Trip report: Summer 2021 ISO C++ standards meeting (virtual)

On Monday, the ISO C++ committee held its third full-committee (plenary) meeting of the pandemic and adopted a few more features and improvements for draft C++23.

We had representatives from 17 voting nations at this meeting: Austria, Bulgaria, Canada, Czech Republic, Finland, France, Germany, Israel, Italy, Netherlands, Poland, Russia, Slovakia, Spain, Switzerland, United Kingdom, and United States. Slovakia is our newest national body to officially join international C++ work. Welcome!

We continue to have the same priorities and the same schedule we originally adopted for C++23, but online via Zoom during the pandemic.

This week: A few more C++23 features adopted

This week we formally adopted a third round of small features for C++23, as well as a number of bug fixes. Below, I’ll list some of the more user-noticeable changes and credit all those paper authors, but note that this is far from an exhaustive list of important contributors… even for these papers, nothing gets done without help from a lot of people and unsung heroes, so thank you first to all of the people not named here who helped the authors move their proposals forward! And thank you to everyone who worked on the adopted issue resolutions and smaller papers I didn’t include in this list.

P1938  by Barry Revzin, Richard Smith, Andrew Sutton, and Daveed Vandevoorde adds the if consteval feature to C++23. If you know about C++17 if constexpr and C++20 std::is_constant_evaluated, then you might think we already have this feature under the spelling if constexpr (std::is_constant_evaluated())… and that’s one of the reasons to add this feature, because that code actually doesn’t do what one might think. See the paper for details, and why we really want if consteval in the language.

P1401 by Andrzej Krzemieński enables testing integers as booleans in static_cast and if constexpr without having to cast the result to bool first (or test against zero). This is a small-but-nice example of removing redundant ceremony to help make C++ code that much cleaner and more readable.

P1132 by Jean-Heyd Meneide, Todor Buyukliev, and Isabella Muerte add out_ptr and inout_ptr abstractions to help with potential pointer ownership transfer when passing a smart pointer to a function that is declared with a T** “out” parameter. In a nutshell, if you’ve ever wanted to call a C API by writing something like some_c_function( &my_unique_ptr ); then these types will likely help you. The idea is that a call site can use one of these types to wrap a smart pointer argument, and then when the helper type is destroyed it automatically updates the pointer it wraps (using a reset call or semantically equivalent behavior).

P1659 by Christopher DiBella generalizes the C++20 starts_with and ends_with on string and string_view by adding the general forms ranges::starts_with and ranges::ends_with to C++23. These can work on arbitrary ranges, and also answer questions such as “are the starting elements of r1 less than the elements of r2?” and “are the final elements of r1 greater than the elements of r2?”.

P2166 by Yuriy Chernyshov helps reduce a commonly-taught pitfall with std::string. You know how since forever (C++98) you can construct a string from a string literal, like std::string("xyzzy")? But that you’d better watch out (and you’d better not cry or pout) not to pass a null pointer, like std::string(nullptr), because that’s undefined behavior where implementations aren’t required to check the pointer for null and can do just whatever they liked, including crash? That’s still the case if you pass a pointer variable whose value is null (sorry!), but with this paper, as of C++23 at least now we have overloads that reject attempts to construct or assign a std::string from nullptr specifically, as a compile-time “d’oh! don’t do that.”

We also adopted a number of other issue resolutions and small papers that made additional improvements, including a number that will be backported retroactively to C++20. Quite a few were of the “oh, you didn’t know that rare case didn’t work? now it does” variety.

Other progress

We also approved work on a second Concurrency TS. Recall that a “TS” or “Technical Specification” is like doing work in a feature branch, which can later be merged into the C++ standards (trunk).

Two related pieces of work were approved to go into the Concurrency TS: P1121 and P1122 by Paul McKenney, Maged M. Michael, Michael Wong, Geoffrey Romer, Andrew Hunter, Arthur O’Dwyer, Daisy Hollman, JF Bastien, Hans Boehm, David Goldblatt, Frank Birbacher, Erik Rigtorp, Tomasz Kamiński, and Jens Maurer add support for hazard pointers and read-copy-update (RCU) which are useful in highly concurrent applications.

What’s next

We’re going to keep meeting virtually in subgroups, and then have at least one more virtual plenary session to adopt features into the C++23 working draft in October.

The next tentatively planned ISO C++ face-to-face meeting is February 2022 in Portland, OR, USA. (Per our C++23 schedule, this is the “feature freeze” deadline for design-approving new features targeting the C++23 standard, whether the meeting is physical or virtual.) Meeting in person next February continues to look promising – barring unexpected surprises, it’s possible that by that time most ISO C++ participating nations will have been able to resume local sports/theatre/concert events with normal audiences, and removed travel restrictions among each other, so that people from most nations will be able to participate at an in-person meeting. But we still have to wait and see… we likely won’t know for sure until well into the autumn, and so we’re still calling this one “tentative” for now. You can find a list of our meeting plans on the Upcoming Meetings page.

Thank you again to the hundreds of people who are working tirelessly on C++, even in our current altered world. Your flexibility and willingness to adjust are much appreciated by all of us in the committee and by all the C++ communities! Thank you, and see you on Zoom.

GotW #102 Solution: Assertions and “UB” (Difficulty: 7/10)

This special Guru of the Week series focuses on contracts. Now that we have considered assertions, postconditions, and preconditions in GotWs #97-101, let’s pause and reflect: To what extent does a failed contract imply “UB”… either the Hidden Dragon of Undefined Behavior, or the Crouching Tiger of Unspecified Behavior?

1. Briefly, what is the difference among:

(a) undefined behavior

Undefined behavior is what happens when your program tries to do something whose meaning is not defined at all in the C++ standard language or library (illegal code and/or data). A compiler is allowed to generate an executable that does anything at all, from data corruption (objects not meeting the requirements of their types) to injecting new code to reformat your hard drive if the program is run on a Tuesday, even if there’s nothing in your source code that could possibly reformat anything. Note that undefined behavior is a global property — it always applies not only to the undefined operation, but to the whole program. [1]

(b) unspecified behavior

Unspecified behavior is what happens when your program does something for which the C++ standard doesn’t document the results. You’ll get some valid result, but you won’t know what the result is until your code looks at it. A compiler is not allowed to give you a corrupted object or to inject new code to reformat your hard drive, not even on Tuesdays.

(c) implementation-defined behavior

Implementation-defined behavior is like unspecified behavior, where the implementation additionally is required to document what the actual result will be on this particular implementation. You can’t rely on a particular answer in portable code because another implementation could choose to do something different, but you can rely on what it will be on this compiler and platform.

2. For each of the following, write a short function … where if the assertion is not checked and is false then the effect:

(a) is always undefined behavior

Easy peasy! Let’s dereference a null pointer:

// Example 2(a): If assert is violated, always undefined behavior

void deref_and_set( int* p ) {
    assert( p );
    *p = 42;
}

The function asserts that p is not null, and then on the next line unconditionally dereferences p and scribbles over the location it points to. If p is null and the assertion checking is off so that we can get to the next line, the compiler is allowed to make running the whole program format our hard drive.

(b) possibly results in undefined behavior

A general way to describe this class of program is that the call site has two bugs: first, it violates a precondition (so the callee’s results are always at least unspecified), and then it additionally then uses the unspecified result without checking it and/or in a dangerous way.

To make up an example, let’s bisect a numeric range:

// Example 2(b): If assert is violated, might lead to undefined behavior

int midpoint( int low, int high ) {
    assert( low <= high );
    return low + (high-low)/2;
        // less overflow-prone than “(low+high)/2”
        // more accurate than “low/2 + high/2”
}

The author of midpoint could have made the function more robust to take the values in either order, and thus eliminated the assertion, but assume they had a reason not to, as alluded to in the comments.

Violating the assertion does not result in undefined behavior directly. The function just doesn’t specify (ahem!) its results if call sites call it in a way that violates the precondition the assertion is testing. If the precondition is violated, then the function can add a negative number to low. But just calculating and returning some other int is not (yet) undefined behavior.

For many call sites, a bad call to midpoint won’t lead to later undefined behavior.

However, it’s possible that some call site might go on to use the unspecified result in a way that does end up being real undefined behavior, such as using it as an array index that performs an out-of-bounds access:

auto m = midpoint( low_index(arr1), high_index(arr2) );   // unspecified
   // here we expect m >= low_index(arr1) ...
stats[m-low_index(arr1)]++;                 // --> potentially undefined

This call site code has a typo, and accidentally mixes the low and high indexes of unrelated containers, which can violate the precondition and result in an index that is less than the “low” value. Then in the next line it tries to use it as an offset index into an instrumentation statistics array, which is undefined behavior for a negative number.

GUIDELINE: Remember that an unspecified result is not in itself undefined behavior, but a call site can run with it and end up with real undefined behavior later. This happen particularly when the calculated value is a pointer, or an integer used as an array index (which, remember, is basically the same thing; a pointer value is just an index into all available memory viewed as an array). If a program relies on unspecified behavior to avoid performing undefined behavior, then it has a path to undefined behavior, and so unspecified behavior is a Crouching Tiger, if you will… still dangerous, and can be turned into to the full dragon.

GUIDELINE: Don’t specify your function’s behavior (output postconditions) for invalid inputs (precondition violations), except for defense in depth (see Example 2(c)). By definition, if a function’s preconditions are violated, then the results are not specified. If you specify the outputs for precondition violations, then (a) callers will depend on the outputs, and (b) those “preconditions” aren’t really preconditions at all.

While we’re at it, here’s a second example: Let’s compare pointers in a way the C++ standard says is unspecified. This program attempts to use pointer comparisons to see whether a pointer points into the contiguous data stored in a vector, but this technique doesn’t work because today’s C++ standard only specifies the results of raw pointer comparison when the pointers point at (into, or one-past-the-end of) the same allocation, and so when ptr is not pointing into v’s buffer it’s unspecified whether either pointer comparison in this test evaluates to false:

// Example 2(b)(ii): If assert is violated, might lead to undefined behavior

// std::vector<int> v = ...;
assert(&v[0] <= ptr && ptr < (&v[0])+v.size());           // unspecified
*ptr = 42;                                  // --> potentially undefined

(c) is never undefined or unspecified behavior

An assertion violation is never undefined behavior if the function specifies what happens in every case even when the assertion is violated. Here’s an example mentioned in my paper P2064, distilled from real-world code:

// Example 2(c): If assert is violated, never undefined behavior
//               (function documents its result when x!=0)

some_result_value DoSomething( int x ) {
    assert( x != 0 );
    if    ( x == 0 ) { return error_value; }
    return sensible_result(x);
}

The function asserts that the parameter is not zero, to express that the call site shouldn’t do that, in a way the call site can check and test… but then it also immediately turns around and checks for the errant value and takes a well-defined fallback path anyway even if it does happen. Why? This is an example of “defense in depth,” and can be a useful technique for writing robust software. This means that even though the assertion may be violated, we are always still in a well-defined state and so this violation does not lead to undefined behavior.

GUIDELINE: Remember that violating an assertion does not necessarily lead to undefined behavior.

GUIDELINE: Function authors, always document your function’s requirements on inputs (preconditions). The caller needs to know what inputs are and aren’t valid. The requirements that are reasonably checkable should be written as code so that the caller can perform the checks when testing their code.

GUIDELINE: Always satisfy the requirements of a function you call. Otherwise, you are feeding “garbage in,” and the best you can hope for is “garbage out.” Make sure your code’s tests includes verifying all the reasonably checkable preconditions of functions that it calls.

Writing the above pattern has two problems: First, it repeats the condition, which invites copy/paste errors. Second, it makes life harder for static analysis tools, which often trust assertions to be true in order to reduce false positive results, but then will think the fallback path is unreachable and so won’t properly analyze that path. So it’s better to use a helper to express the “either assert this or check it and do a fallback operation” in one shot, which always avoids repeating the condition, and could in principle help static analysis tools that are aware of this macro (yes, it would be nicer to do it without resorting to a macro, but it’s annoyingly difficult to write the early return without a macro, because a return statement inside a lambda doesn’t mean the same thing):

// Using a helper that asserts the condition or performs the fallback

#define ASSERT_OR_FALLBACK(B, ACTION) { \
    bool b = B;                         \
    assert(b);                          \
    if(!b) ACTION;                      \
}

some_result_value DoSomething( int x ) {
    ASSERT_OR_FALLBACK( x != 0, return error_value; );
    return sensible_result(x);
}

3. Explain how your answers to Questions 1 and 2 do, or do not, correspond with each other.

In Example 2(a), violating the assertion leads to undefined behavior, 1(a).

In Example 2(b), violating the assertion leads to unspecified behavior, 1(b). At buggy call sites, this could subsequently lead to undefined behavior.

In Example 2(c), violating the assertion leads to implementation-defined behavior, 1(c), which never in itself leads to  undefined behavior.

4. BONUS: Describe a valuable service that a tool could perform for assertions that satisfy the requirement in 2(a), that is not possible for other assertions.

There are many. Here is just one example, that happens to be nice because it is perfectly accurate.

Let’s say we have all the code examples in question 2, written using C assert today (or even with those assertions missing!), and then at some future time we get a version of standard C++ that can express them as preconditions. Then only in Example 2(a), where we can see that the function body (and possibly transitively its further callees with the help of inlining) exercises undefined behavior, a tool can infer the precondition annotation and add it mechanically, and get the benefit of diagnosing existing bugs at call sites:

// What a precondition-aware tool could generate for Example 2(a)

auto f( int* p ) 
    [[pre( p )]]  // can add this automatically: because a violation
                  // leads to undefined behavior, this precondition
                  // is guaranteed to never cause a false positive
{
    assert( p );
    *p = 42;
}

For example, after some future C++2x ships with contracts, a vendor could write an automated tool that goes through every open source C++ project on GitHub and mechanically generates a pull request to insert preconditions for functions like Example 2(a) – but not (b) or (c) – whether or not the assertion already exists, just by noticing the undefined behavior. And it can inject those contract preconditions with complete confidence that none of them will ever cause a false positive, that they will purely expose existing bugs at call sites when that call site is built with contract checking enabled. I would expect such tool to identify a good number of (at least latent if not actual) bugs, and be a boon for C++ users, and it’s possible only for functions in the category of 2(a).

“Automated adoption” of at least part of a new C++ feature, combined with “automatically identifies existing bugs” in today’s code, is a pretty good value proposition.

Acknowledgments

Thank you to the following for their comments on this material: Joshua Berne, Gabriel Dos Reis, Gábor Horváth, Andrzej Krzemieński, Ville Voutilainen.

Notes

[1] In the standard, there are two flavors of undefined behavior. The basic “undefined behavior” is allowed to enter your program only once you actually try to execute the undefined part. But some code is so extremely ill-formed (with magical names like “IF-NDR”) that its very existence in the program makes the entire program invalid, whether you try to execute it or not.

GotW #102: Assertions and “UB” (Difficulty: 7/10)

This special Guru of the Week series focuses on contracts. Now that we have considered assertions, postconditions, and preconditions in GotWs #97-101, let’s pause and reflect: To what extent does a failed contract imply “UB”… either the Hidden Dragon of Undefined Behavior, or the Crouching Tiger of Unspecified Behavior?

JG Question

1. Briefly, what is the difference among:

(a) undefined behavior

(b) unspecified behavior

(c) implementation-defined behavior

Guru Questions

2. For each of the following, write a short function of the form:

/*...function name and signature...*/
{
    assert( /*...some condition about the parameters...*/ );
    /*...do something with parameters...*/;
}

where if the assertion is not checked and is false then the effect:

(a) is always undefined behavior

(b) possibly results in undefined behavior

(c) is never undefined or unspecified behavior

3. Explain how your answers to Questions 1 and 2 do, or do not, correspond with each other.

4. BONUS: Describe a valuable service that a tool could perform for assertions that satisfy the requirement in 2(a), that is not possible for other assertions.

GotW #101 Solution: Preconditions, Part 2 (Difficulty: 7/10)

This special Guru of the Week series focuses on contracts. We covered some basics of preconditions in GotW #100. This time, let’s see how we can use preconditions in some practical examples…

1. Consider these functions, expanded from an article by Andrzej Krzemieński: [1] … How many ways could a caller of each function get the arguments wrong, but that would silently compile without error? Name as many different ways as you can.

There are several ways to break this down. I’ll use three major categories of possible mistakes, the first two of which overlap:

  • wrong order: passing an argument in the wrong position
  • wrong value: passing an argument with a valid but wrong value (e.g., index out of range)
  • invalid value: passing an argument that is already invalid (e.g., an invalid iterator)

Let’s see how these play out with our three examples, starting with (a).

(a) is_in_values (int val, int min, int max)

// Example 1: Adapted from [1]

auto is_in_values (int val, int min, int max)
  -> bool;  // true iff val is in the values [min, max]

Oh my, three identically typed integer parameters… what could be confusing about that?!

Wrong order (5 ways): First, there are five ways to pass these in the wrong order, because there are 3! = 6 permutations, all of which compile but only the first of which is correct:

is_in_values( v,  lo, hi );    // correct

is_in_values( v,  hi, lo );    // all these are wrong, but compile :(
is_in_values( lo, v,  hi ); 
is_in_values( lo, hi, v  ); 
is_in_values( hi, v,  lo ); 
is_in_values( hi, lo, v  );

Some of these argument orders may seem strange, but some are orders other libraries’ similar APIs might use which makes confusion easier, we all make mistakes… and the type system isn’t helping us at all.

Wrong value (1 way): Second, there is an implicit precondition that min <= max, so passing arguments where min > max would be wrong, but would silently compile. Some of these are exercised by the “wrong order” permutations above, but even call sites that remember the right argument order can make mistakes about the actual values.

Invalid value (0 ways): Finally, all possible values of an int are valid — some may be suspiciously big or small, but int doesn’t have the concept of “not a number” (NaN) as we have with floats, or the concept of “invalidated” like we have with iterators.

(b) is_in_container (int val, int idx_min, int idx_max)

It sure doesn’t help that the next function has the identical signature as is_in_values, but with very different meaning:

auto is_in_container (int val, int idx_min, int idx_max)
  -> bool; // true iff container[i]==val for some i in [idx_min, idx_max]

Wrong order (5 ways): As in (a), we again have five ways to pass these in the wrong order, all of which compile but only the first is correct:

is_in_container( v,  lo, hi );    // correct

is_in_container( v,  hi, lo );    // all these are wrong, but compile :(
is_in_container( lo, v,  hi ); 
is_in_container( lo, hi, v  ); 
is_in_container( hi, v,  lo ); 
is_in_container( hi, lo, v  );

Wrong value (3 ways): Again as in (a), we have the implicit precondition that idx_min <= idx_max, so passing idx_min > idx_max would be wrong, but would silently compile. But this time there are two additional ways to go wrong, because idx_min and idx_max must both be valid subscripts into container, so if either is outside the range [0, container.size()) it is a valid integer but an out of bounds value for this use.

Invalid value (0 ways): Again as in (a), all possible values of an int are valid — though some may be wrong values if they’re out of bounds as we noted above, they’re still valid integers.

(c) is_in_range (T val, Iter first, Iter last)

template <typename T, typename Iter>
auto is_in_range (T val, Iter first, Iter last)
  -> bool; // true iff *i==val for some i in [first,last)

Wrong order (1 way): This time there’s only one way to pass the parameters in the wrong order (ignoring pathological cases where the same argument might convert both T and Iter):

is_in_container( v, istart, iend );    // correct

is_in_container( v, iend, istart );    // wrong, but compiles :(

Wrong value (2 ways): We could pass a first and last that are not a valid range in two ways:

  • they point into the same container, but first doesn’t precede last
  • they point into different containers

Invalid value (2 ways): And finally, either of first or last could actually be an invalidated iterator (e.g., dangling). For example, the container they point into may be destroyed so that both are invalid; or one of the two iterators might have been calculated before a more recent operation like vector::push_back that could have invalidated it.

But if the sight of these function signatures has had you pulling your hair and shouting “use the type system, Luke!” at your screen, you’re not alone… now let’s make things better.

2. Show how can you improve the function declarations in Question 1 by …

(a) just grouping parameters, using a struct with public variables

Interestingly, we actually get a lot of benefit simply by grouping ‘parameters that go together,’ using an creating an aggregate or “grouping” helper struct.[3] For example:

// Example 2(a)(i): Improving Example 1 with aggregate types

struct min_max { int min, max; };

auto is_in_values (int val, min_max minmax) -> bool;
auto is_in_container (int val, min_max rng) -> bool;

template <typename Iter> struct two_iters { Iter first, last; };

template <typename T, typename Iter>
auto is_in_range (T val, two_iters<Iter> rng) -> bool;

Or even just venerable anonymous std::pair is better than no grouping:

// Example 2(a)(ii): Improving Example 1 with aggregate types

auto is_in_values (int val, std::pair<int,int> minmax) -> bool;
auto is_in_container (int val, std::pair<int,int> rng) -> bool;

template <typename T, typename Iter>
auto is_in_range (T val, std::pair<Iter,Iter> rng) -> bool;

With either of the above, there’s only one way for callers to get the argument order wrong. And it requires only two extra characters at call sites, because we can use { } to group the arguments without creating actual named objects of the helper struct:

is_in_values( v, {lo, hi} );	// correct
is_in_values( v, {hi, lo} );	// wrong, but compiles

is_in_container( v, {lo, hi} );	// correct
is_in_container( v, {hi, lo} );	// wrong, but compiles

is_in_range( v, {i1, i2} );		// correct
is_in_range( v, {i2, i1} ); 	// wrong, but compiles

So just grouping parameters using a struct eliminates some errors. But really using the type system is even better…

(b) just using an encapsulated class, using a class with private variables (an abstraction with its own invariant)

Clearly all three functions are crying out for a “range”-like abstraction for its pair of parameters, in the first two cases a range of values and in the third a range of iterators. How do we know? Because:

Here’s one way we can apply class types we can find in the standard library or Boost today:

// Example 2(b): Improving Example 1 with encapsulated class types

auto is_in_values (int val, boost::integer_range<int> rng) -> bool;

auto is_in_container (int val, boost::integer_range<int> rng) -> bool;

template <typename T, std::ranges::input_range Range>
auto is_in_range (T val, Range&& rng) -> bool;

This gives us all the mistake-reduction goodness we got in (a), plus more.

First, as in (a), absent pathological conversions, it’s very difficult to get arguments in the wrong order simply because of being forced to group the parameters:

auto minmax = boost::irange(10, 100);
is_in_values( 42, minmax );

auto minmax2 = boost::irange(0, ssize(myvec)-1);
is_in_container( 42, minmax2 );

auto myvec = std::vector<int>();
is_in_range( 42, myvec );

But, unlike our helper structs in (a), we now get additional safety because the types can express constructor preconditions that move some of those mistakes (such as (hi,lo) misordering) to constructors of class abstractions that can then preserve them as invariants [4] – so the mistake can still be made but in fewer places, to where we construct or modify the abstracted object (e.g., range), rather than every time we use un-abstracted separately values (e.g., a couple of iterator objects we have lying around and whose relationship we have to maintain by hand over time). This is why we sometimes say “types are predicates,” because a type encapsulates a predicate, namely its invariant.

GUIDELINE: When multiple functions state the same precondition, it’s a telltale sign there’s a missing class that should turn it into an invariant. A repeated precondition is nearly always a “naked invariant” that should be encapsulated up inside a type. This is more obvious when the precondition involves multiple parameters (or ordinary variables for that matter); a poster child is the STL’s pervasive use of iterator pairs, which have long been crying out to be encapsulated using a range abstraction, and fortunately we now have that in C++20. Consider using a class instead.

GUIDELINE: Remember that a key reason why encapsulated classes are powerful is that they wrap up preconditions and turn them into invariants. Hiding data members is good dependency management because it limits the code that can depend on the details of the data and is responsible for maintaining the correct relationship among the data members.

(c) just using post-C++20 contract preconditions (not yet valid C++, but something like the syntax in [2])

Preconditions test values, so they can let us eliminate the “wrong values” kinds of mistakes. Consider this code:

// Example 2(c): Improving Example 1 with boolean preconditions

auto is_in_values (int val, int min, int max)
  -> bool // true iff val is in the values [min, max]
     [[pre (min <= max)]]
;

auto is_in_container (int val, int idx_min, int idx_max)
  -> bool // true iff container[i]==val for i in [idx_min, idx_max]
     [[pre (0       <= idx_min
         && idx_min <= idx_max
         && idx_max <  container.size())]]          // see note [5]
;

template <typename T, typename Iter>
auto is_in_range (T val, Iter first, Iter last)
  -> bool // true iff *i==val for some i in [first,last)
     [[pre (/*... is_reachable? is_not_dangling? hmm ...*/)]]
;

For the first two functions, we can write clear preconditions that can check the “wrong value” bugs.

In these particular examples, the best place to write the preconditions is right on the constructors of the class types we saw in (b), and if we write them there then we don’t have to repeat them as explicit contracts on every function.

But is (b) always better than (c), in other examples? This brings us to our last question, which is all about “can” versus “should”…

3. Consider these three examples, where each shows expressing a boolean condition either as a function precondition or as an encapsulated invariant inside a new type… In each of these cases, which way is better? Explain your answer.

In Question 2, writing a type was often the best choice, but it isn’t always.

The benefits to writing a type include:

  • Encapsulation. We limit the code that is responsible for maintaining the boolean condition.
  • Language support. We get the help of the type system to statically enforce requirements.

But there are costs and limitations too:

  • What’s the abstraction? There may not be a suitable one. We can’t write a good type unless we can discover a useful abstraction that the type’s interface should support. A good type represents a useful reusable domain abstraction that programmers can understand and that makes their code clearer by elevating the vocabulary of the code. There won’t always be a practical and reusable abstraction; when there isn’t, we won’t be able to write a useful and reusable type. — Even when there is, we have to design that all ahead of time, which requires a lot more advance knowledge and engineering than just writing ad-hoc boolean conditions on individual functions.
  • What’s the cost? It may not be feasible to maintain the invariant. We have to do any extra work it takes to maintain the invariant, and it has to be practical to do. When it isn’t, we can’t maintain the invariant without help from outside code, and so we won’t be able to really encapsulate it properly.
  • Does it make sense as an independent abstraction? Will the user be carrying around objects of this type, or are we just jamming a precondition common to a few functions (or only one) into a type and calling it useful? Occam’s Razor: Don’t multiply entities beyond necessity.
  • What’s the type the caller is using? This is where a real usable abstraction shines, because many callers will be using it independently of calling our function. But if the caller isn’t using this type, then there typically has to be an implicit or explicit conversion (because inheritance from all argument types our callers might already have usually isn’t an option), and that conversion would need to be usable and sufficiently cheap.

GUIDELINE: Remember that types and contracts are “better together.” Use both. They are complementary, neither is a substitute for the other. All we are trying to accomplish with contracts is to augment the language’s static type checking with runtime checking where that is more appropriate because we can’t design a practical abstraction. And this is why we want contracts on functions (preconditions, postconditions) even though we already have types, and why we also want contracts on types (invariants).

Let’s consider the three examples.

(a) A vector that is sorted

template <typename T>
void f( vector<T> const& v ) [[pre( is_sorted(v) )]] ;

template <typename T>
void f( sorted<vector<T>> const& v );

If this looks familiar, it’s because is_sorted is one of the classic examples we saw in GotW #98 of conditions that are often impractical to check and enforce as an assertion, in this case a precondition.

Can we do better by making it a type, perhaps a sorted wrapper around a container like vector that maintains the guarantee that it’s always sorted? Well, we have to answer some questions about a sorted<T>:

  • What’s the abstraction it provides? It can’t easily fulfill the requirements of a sequence container like vector itself; for example, push_back doesn’t make much sense because letting the caller insert an arbitrary value at a specific location would easily cause the container to be unsorted. Instead, it would naturally want a more general insert function instead, and the interface would be more like set. This part could be workable.
  • What’s the cost? This where it starts to breaks down: Keeping a vector sorted all the time means that every insertion would cost O(N) work all the time. Which leads into…
  • Does it make sense as an independent abstraction? … that it’s very common for code to maintain an “almost-sorted” vector, such as by inserting new elements at the end which is fast (and, hmm, affects our abstraction design, because then it would make sense to have push_back after all, wouldn’t it? hmm) but leaves a suffix of unsorted elements in the container, and then periodically sorting the whole container so that the sorting cost is amortized. But an almost-sorted vector isn’t good enough, and so doesn’t fit the bill. We don’t have empirical evidence of such types in general use.
  • What’s the type the caller is using? And now we’re busted all the way, because we want this interface to be usable by anyone who has a vector<T>, which would require a conversion to sorted<vector<T>>. If we do a deep copy, that’s prohibitively expensive. Even if the conversion is lightweight by avoiding a deep copy, such as by just wrapping an existing vector object, it wouldn’t be very useful unless it did O(N) work every time unconditionally to verify the invariant. And even then the abstraction design is affected and compromised: If the user can still see and modify the original vector, then that’s still part of the accessible interface to the data, so the user can make the container be not fully sorted and we’re unable to really encapsulate and maintain our intended invariant.

So is_sorted is much better as a function precondition.

// (b) A vector that is not empty

template <typename T>
void f( vector<T> const& v ) [[pre( !v.empty() )]] ;

template <typename T>
void f( not_empty<vector<T>> const& v );

This one is more feasible as a type, but still not ideal:

  • What’s the abstraction it provides? It’s a vector, and we can make the interface identical to vector with just extra preconditions on pop and erase functions to not remove the last element in the container.
  • What’s the cost? Emptiness is cheap enough to check and maintain.
  • Does it make sense as an independent abstraction? This is where it starts to get questionable… the answer is at best “maybe.” It’s not clear to me than a “nonempty vector” is a generally useful abstraction.
  • What’s the type the caller is using? This is where I think we break down again. Again, we want this interface to be usable by anyone who has a vector<T>, and that means a conversion to not_empty<vector<T>>. If we do a deep copy, that’s prohibitively expensive. This time if we just wrap an existing vector object to avoid the deep copy, the check is cheap. But then we still have the problem that the abstraction design is affected and compromised so that it can’t maintain its invariant, because if the user can still see and modify the original vector, they can remove the last element on us.

So not_empty seems better as a function precondition.

(c) A pointer that is not null

void f( int* p ) [[pre( p != nullptr )]] ;

void f( not_null<int*> p );

This time we can do better:

  • What’s the abstraction it provides? This one’s easy to state: It’s a not-null pointer. That’s a far simpler interface than a container, because we just need operator* and operator->, construction, destruction, and copying. Even so it’s not totally without subtlety, because not_null should not have move operations that modify the source object. This means that a not_null<unique_ptr<T>> is legal but there’s not much you can do with it besides dereference it and destroy it: It can’t be copyable because unique_ptr isn’t copyable, and it must not be movable because moving a unique_ptr leaves the source null.
  • What’s the cost? Nullness is cheap enough to check and maintain.
  • Does it make sense as an independent abstraction? Definitely. A “non-null pointer” has been widely rediscovered and reinvented as a generally useful abstraction.
  • What’s the type the caller is using? A not_null<int*> is a useful object in its own right in the calling code, independently of calling this particular function. And if our function is invoked by someone who has only an ordinary int*, doing a full copy of the pointer is cheap, and applying the nullness check as a precondition on that converting constructor is exactly equivalent to writing the precondition by hand, but is automated.

So not_null seems better as a type, primarily because it is independently useful. This is why it has been reinvented a number of times, including as gsl::not_null. [6]

GUIDELINE: Wherever practical, design interfaces so that incorrect call sites are illegal (won’t compile, using the type system) or loud (won’t pass unit tests, using preconditions). This is a key part of achieving the goal to “make interfaces easy to use correctly, and hard to use incorrectly.” Preconditions directly help with that by letting us catch entire groups of errors at test time, and are a complement to the type system which makes incorrect uses “not fit” through the compiler and also carries extra preconditions around for us in the form of invariants.

GUIDELINE: Remember that the type system is a hammer, and not every precondition is a nail. The type system is a powerful tool, but not every precondition is naturally (part of) an invariant of a useful type that provides a good reusable abstraction that’s generally useful independently of this function.

Notes

[1] A. Krzemieński. “Contracts, preconditions and invariants” (Andrzej’s C++ blog, December 2020).

[2] G. Dos Reis, J. D. Garcia, J. Lakos, A. Meredith, N. Myers, and B. Stroustrup. “P0542: Support for contract based programming in C++” (WG21 paper, June 2018). Subsequent EWG discussion favored changing “expects” to “pre” and “ensures” to “post,” and to keep it as legal compilable (if unenforced) C++20 for this article I also modified the syntax from : to ( ), and to name the return value _return_ for postconditions. That’s not a statement of preference, it’s just so the examples can compile today to make them easier to check.

[3] For 2(a) and 2(b), on platform ABIs that do not pass small structs/classes in registers, turning individual parameters into a struct/class could cause them to be passed in stack memory instead of in registers.

[4] Upcoming GotWs will cover invariants and violation handling.

[5] If C++ gets chained comparisons as proposed in P0515 and P0893 we could write this much more clearly, and with fewer opportunities for mistakes, as:

[[pre( 0 <= idx_min <= idx_max < container.size() )]]

[6] B. Stroustrup and H. Sutter (eds.) “I.12 Declare a pointer that must not be null as not_null” (C++ Core Guidelines.) If the not_null<T> type we are using is implicitly convertible from T, which is the intent of I.12 to provide a drop-in replacement for pointer parameters, then the usability is the same as with the precondition. Otherwise, the caller has to provide a not_null argument at the call site, either by doing an explicit conversion or by just using a not_null local variable in their own body.

Acknowledgments

Thank you to the following for their feedback on this material: Joshua Berne, Gabriel Dos Reis, J. Daniel Garcia, Gábor Horváth, Andrzej Krzemieński, Bjarne Stroustrup, Andrew Sutton, Ville Voutilainen

Trip report: Winter 2021 ISO C++ standards meeting (virtual)

Today, the ISO C++ committee held its second full-committee (plenary) meeting of the pandemic and adopted a few more features and improvements for draft C++23.

A record of 18 voting nations sent representatives to this meeting: Austria, Bulgaria, Canada, Czech Republic, Finland, France, Germany, Israel, Italy, Japan, Netherlands, Poland, Romania, Russia, Spain, Switzerland, United Kingdom, and United States. Japan had participated in person during C++98 and C++11, and has always given us good remote ballot feedback during C++14/17/20, and is attending again now; welcome back! Italy and Romania are our newest national bodies; welcome!

Our virtual 2021

We continue to have the same priorities and the same schedule we originally adopted for C++23. However, since the pandemic began, WG21 and its subgroups have had to meet all-virtually via Zoom, and we are not going to try to have a face-to-face meeting in 2021 (see What’s Next below). Some subgroups had already been having virtual meetings for years, but this was a major change for other groups including our two main design groups – the language and library evolution working groups (EWG and LEWG). In all, over the past year we have held approximately 200 virtual meetings.

Today: A few more C++23 features adopted

Today we formally adopted a second round of small features for C++23, as well as a number of bug fixes. Below, I’ll list some of the more user-noticeable changes and credit all those paper authors, but note that this is far from an exhaustive list of important contributors… even for these papers, nothing gets done without help from a lot of people and unsung heroes, so thank you first to all of the people not named here who helped the authors move their proposals forward! And thank you to everyone who worked on the adopted issue resolutions and smaller papers I didn’t include in this list.

P1102 by Alex Christensen and JF Bastien is the main noticeable change we adopted for the core language itself. It’s just a tiny bit of cleanup, but one that I’m personally fond of: In C++23 we will be able to omit empty ( ) lambda parameter lists even when we have to declare the lambda mutable. I’m the one who proposed the lambda syntax we have today (except for the mutable part which wasn’t mine and I never liked), including that it enabled making unused parts of the syntax optional so that we can write simple lambdas simply. For example, today we can already write

[x]{ return f(x); }

as a legal synonym for

[x] () -> auto { return f(x); }

and omit the empty parameter list and deduced return type. Even so, I’ve noticed a lot of people write the ( ) part anyway, which isn’t wrong or anything, it’s just that often they write it because they don’t know they can omit it too. And part of the problem was the oddity in pre-C++23 that if you need to write mutable, then you actually do have to also write the ( ) (but not the return type), which was just weird but was another reason for people to just write ( ) all the time, because sometimes they had to. With P1102, we don’t have to. That’s more consistent. Thanks, Alex and JF!

In the spirit of “completing C++20,” P2259 by Tim Song makes several fixes to iterator_category to make it work better with ranges and adaptors. Here is an example of code that does not compile today for arcane reasons (see the paper), but will be legal C++23 thanks to Tim:

std::vector<int> vec = {42};
auto r = vec | std::views::transform([](int c) { return std::views::single(c);})
             | std::views::join
             | std::views::filter([](int c) { return c > 0; });
r.begin();

Further in the “completing C++20” spirit, P2017 by Barry Revzin fixes some additional glitches in ranges to make them work better. Here is an example of safe and efficient code that does not compile today, where for arcane reasons the declaration of e isn’t supported and today’s workaround is to make the code more complex and less efficient. This will be legal C++23 thanks to Barry:

auto trim(std::string const& s) {
    auto isalpha = [](unsigned char c){ return std::isalpha(c); };
    auto b = ranges::find_if(s, isalpha);
    auto e = ranges::find_if(s | views::reverse, isalpha).base();
    return subrange(b, e);
}

P2212 by Alexey Dmitriev and Howard Hinnant generalizes time_point::clock to allow for greater flexibility in the kinds of clocks it supports, including stateful clocks, external system clocks that don’t really have time_points, representing “time of day” as a distinct time_point, and more.

P2162 by Barry Revzin takes an important first step toward cleaning up std::visit and lay the groundwork for its further generalization. Even if you don’t yet love std::visit, it’s a useful tool that P2162 makes more useful by making it work more regularly. We expect to see further generalization in the future, which is much easier to do with a cleaner and more regular existing feature to build upon.

Finally, I saw cheers and celebratory emoji erupt in the Zoom chat window when we adopted P1682 by JeanHeyd Meneide. It’s very small, but very useful. When passing an enum to an API that uses the underlying type, today we have to write a static_cast to the std::underlying_type, which makes us repeat the enum’s name and so is cumbersome all the time and brittle for type-safety under maintenance if we change to use a different enum:

some_untyped_api( static_cast<std::underlying_type_t<ABCD>>(some_value) );

Thanks to JeanHeyd, in C++23 we will be able to write:

some_untyped_api( std::to_underlying(some_value) );

Note that of course standard library vendors don’t have to wait until 2023 to provide to_underlying or any of these other fixes and improvements. Just having a feature like this one voted into the draft standard is often enough for vendors to be proactive in providing it… these days, vendors are more closely tracking our draft standard meeting by meeting rather than waiting for the official release, in part because we are shipping regularly and predictably and we don’t vote features into the draft standard until we think they’re pretty well baked so that vendors have less risk in implementing them early.

We also adopted a number of other issue resolutions and small papers that made additional improvements.

Finally, we came close to adopting P0533 by Edward Rosten and Oliver Rosten, which is about adding constexpr to many of the functions in math.h that we share with C. This is clearly a Good Thing and therefore many voted in favor of adopting the paper. The only hesitation that stopped it from getting consensus this time were concerns that it needed more time to iron out how implementations would implement it, such as how to deal with errno in a constexpr context. This is the kind of question that often arises when we want to make improvements to entities declare in the C headers, because not only are they governed by the C standard rather than the C++ standard, but typically they are provided and controlled by the operating system vendor rather than by the C++ compiler/library writer, and those constraints always mean a bit of extra work when we want to make improvements for C++ programmers and remain compatible. As far as I know, everyone wants to see these functions made constexpr, so we expect to see this paper come to plenary again in the future. Thanks for your perseverance, Edward and Oliver!

What’s next

As long as we are meeting virtually, we will continue to have virtual plenaries like the one we had this week to formally adopt new features as they progress through subgroups. Our next two virtual plenaries to adopt features into the C++23 working draft will be held in June and November. Progress will be slower than when we can meet face-to-face, and we’ll doubtless defer some topics that really need in-person discussion until we can meet again safely, but in the meantime we’ll make what progress we can and we’ll ship C++23 on time.

The next tentatively planned face-to-face meeting is February 2022 in Portland, OR, USA; however, we likely won’t know until well into the autumn whether we’ll be able to confirm that or need to postpone it. You can find a list of our meeting plans on the Upcoming Meetings page.

Thank you again to the hundreds of people who are working tirelessly on C++, even in our current altered world. Your flexibility and willingness to adjust are much appreciated by all of us in the committee and by all the C++ communities! Thank you, and see you on Zoom.

Firsts in 2020 (or, A little dose of good news)

2020 has been mostly terrible. That includes for the C++ committee and many of our communities, where just this month we lost Beman Dawes. Beman was one of the most important and influential C++ experts in the world, and made his many contributions mostly behind the scenes. I and everyone else who has ever benefited from any of the standardized STL, Boost, C++Now, std::filesystem, C++98/11/14/17, and more — so, really, most people who have ever used C++ — all owe Beman a debt of gratitude. We miss him greatly.

To end the year with a little dose of good news, I thought I’d mention a just few positive C++ accomplishments that did happen for 2020, and were happier “first-ever” achievements.

First, the big one…

C++20 is the first ever “D&E-complete” release of C++. In February, we completed C++20, which is the first release of Standard C++ that includes every feature that Bjarne Stroustrup envisioned for C++’s evolution in his 1994 book The Design and Evolution of C++ (aka D&E), including concepts, coroutines, modules, and more, except only for one minor feature (unified function call syntax). Thank you to Bjarne for sticking with it until we got there, and personally doing the heavy lifting to drive important features like concepts into Standard C++!

C++20 is the first release of C++ that added a feature that made the standard smaller. When I talk about the importance of simplifying C++ by judiciously adding features that let programmers express their intent directly, some people legitimately object that adding a feature makes C++ bigger and more complex. I reply “but it makes C++ code simpler” and “if it replaces something more complex then we can teach a simpler C++ for new code,” but those effects have been hard to measure concretely. Now in C++20 for the first time we added a new feature that made the standard smaller: We added the C++20 spaceship operator to the language, but we also applied it throughout the C++ standard library and that made the library specification nearly 20 pages shorter — a net reduction. So for the first time we can measure that, yes, adding a feature to C++ can make C++ smaller. Thank you to everyone who helped me with that proposal and who are listed in the Acknowledgements in the link, and especially to Walter E. Brown, Jens Maurer, Barry Revzin, and David Stone!

First year for all-virtual standards meetings, including EWG and LEWG. Since March, for the first time all major subgroups including the two main design subgroups of EWG (language) and LEWG (library) have been having virtual meetings by telecon or Zoom and making progress in between face-to-face meetings. We’ve also had a record number of nearly 20 virtual subgroup meetings on average per month. It’s great to see that, despite the pandemic, the committee has continued work on C++23 and other long-pole features, and in November we were able to formally adopt the first C++23 features into our brand-new C++23 working draft. Thank you once again to JF Bastien (EWG), Bryce Lelbach (LEWG), and their assistants, and all the other subgroup chairs and participants for patiently supporting these changes that we had to invent and transition to at short notice, and as we continue to work out the kinks as we go!

Many first virtual conferences. And of course 2020 saw many of our C++ conferences hold their first virtual events (and create new ones like Pure Virtual C++), in the face of huge uncertainties and technical challenges with bleeding-edge technologies, to make it all work far more smoothly than we really would have had any right to expect on such short notice. Thank you to the organizers for working so busily behind the scenes to make it possible to have a facsimile of our face-to-face conferences until we can meet again in person!

Here’s hoping that by this time next year we will all be doing better in every way, and have a happier 2021 to reflect upon. Thank you again, everyone, for your interest in C++ and support for our many C++ events, forums, and other communities large and small, and best wishes for a great 2021.

Trip report: Autumn ISO C++ standards meeting (virtual)

On Monday, the ISO C++ committee completed its final full-committee (plenary) meeting of 2020 and adopted the first changes to the C++23 working draft, including a few new features.

This was a first in several ways: It was our first-ever virtual plenary, held online via Zoom. It was also our first-ever plenary meeting that wasn’t held at the end of a long around-the-clock week of intensive subgroup meetings; instead, it was held at the end of nearly nine months of virtual subgroup meetings.

Our virtual 2020

The pandemic was just getting started when we held our February meeting in Prague, Czech Republic. Since then it has of course been impossible to meet in person; as I mentioned before, our ISO C++ meetings are virtual until further notice, but we continue to have the same priorities and the same schedule for C++23.

So since the pandemic began, WG21 subgroups have been meeting virtually via Zoom. Some subgroups had already been having virtual meetings for years, but this was a major change for other groups including our two main design groups – the language and library evolution working groups (EWG and LEWG).

In all, since Prague we have held about 150 virtual meetings. When lots of subgroups are meeting, some of them weekly, those meetings add up!

This week: First C++23 features adopted

On Monday we formally adopted the first features of C++23, including the first C++23 language feature, as well as a number of bug fixes.

First up was P0330 by JeanHeyd Meneide, which adds a literal suffix for (signed) size_t, so in C++23 we will be able to write literals like 100uz. (I wonder whether uz will be pronounced Uzi.) See the many excellent side-by-side examples in JeanHeyd’s paper for how this helps make uses of size_t safer and more convenient especially in naked for loops iterating over containers. Congratulations to JeanHeyd for C++23’s first language extension, and also for his persistence with this paper – the adopted version is revision 8, and that number and the paper’s change history indicates the level of rigor that can be required to get a feature into C++. Many thanks!

P1679 by Wim Leflere and Paul Fee add a basic_string::contains function so we can write code like if (str.contains(substr)) std::cout << “found!\n”; … I can already hear the chorus of “finally!”

P0881 by Alexey Gorgurov and Antony Polukhin add a stacktrace library to C++23. This is a much-anticipated extension based on Boost.Stacktrace that will enable much easier-to-debug portable diagnostic messages.

The charmingly numbered P1048 by Juan Alday gives us an is_scoped_enum type trait to detect when an enumeration is defined using the new-style (C++11, but well it’s still “new”!) enum class. As the paper points out, this is particularly useful as a migration aid, including to write code that detects and measures the adoption of “enum class” over plain old “enum.”

Finally, P0943 by Hans Boehm supports C atomics (spelled _Atomic) in C++ where the two did not already overlap, which helps write headers that work in both C and C++. (The adopted version is R6 which should be published in the next few weeks.) This is one example of the ongoing extra coordination we’ve had lately between the C and C++ committees, which leads to the next thing we did…

New SG22: C/C++ liaison

We appointed a new study group, SG22, for C/C++ liaison. This is a unique study group, because it is shared jointly by both the C and C++ committees, and it continues the tradition of closer coordination between the two committees. Thank you to WG14 (C) and its chair David Keaton for their continued interest in coordinating the two languages, to Aaron Ballman for agreeing to chair this new group, and for our WG14 and WG21 project editors Thomas Köppe, JeanHeyd Meneide, and Richard Smith to serve as assistant chairs. Thank you all for being willing to step up!

Other updates

Thank you to Richard Smith for his work for many years as project editor for the C++ standard, and completing C++20 this month! Thank you also to the many of you who have helped Richard and shared the editing workload by providing PRs and proofreading to apply plenary resolutions; that has been very much appreciated by Richard and by all of us especially given that C++20 is a “big” release with many new features, all of which have created an unusually high amount of editing work for this release.

Starting now, as we begin C++23, Thomas Köppe has graciously agreed to step up to be our primary project editor for the standard, with Richard as backup project editor. Thank you Thomas, and thank you again Richard and to all who have helped with the editing for the C++ IS!

Next steps

While we are meeting virtually until further notice, we will continue to have virtual plenaries like the one we had this week to formally adopt new features as they progress through subgroups. Our next virtual plenary will be in February, on the Monday of what would have been the Kona meeting.

Progress during this time will be slower than when we can meet face-to-face, and we’ll doubtless defer some topics that really need in-person discussion until we can meet again safely, but in the meantime we’ll make what progress we can and we’ll ship C++23 on time.

Thank you again to the hundreds of people who are working tirelessly on C++, even in our current altered world. Your flexibility and willingness to adjust are much appreciated by all of us in the committee and by all the C++ communities! Thank you, and see you on Zoom.

My plans at CppCon

It’s hard to believe CppCon 2020 is nearly here… in fact, pre-conference tutorials are already in progress.

I’ll be at the conference throughout the week in the hallways and session rooms. Here are some of the times I’ll be participating on the actual program:

  • Sunday 1300 MDT: Organizer’s Panel. In the middle of the Welcome Reception, we’re holding an Organizer’s Panel where several of us organizers will be talking about what to expect in the week ahead and available for extensive Q&A to answer any questions you may have. (We will have live music during the whole Welcome Reception, so be sure to watch the chat window for how to join that stream if you want to listen to our CppCon house band leader, Jim Basnight, perform for us.)
  • Monday 1500 MDT: My AMA. I’ll be available for an “ask me anything” session. Attendees can ask questions across the board from C++20 features, to how the committee is working during the pandemic on C++23, to specific C++ features I’ve designed or contributed to like concurrency or structured bindings or the spaceship operator, or other things you may think of.
  • Tuesday 0730 MDT: Committee Fireside Chat Panel (moderator). This year, I won’t be a panelist myself, so I don’t plan to answer any questions. Instead, I’ll be the panel moderator, and we have a great slate of panelists again this year: Bjarne Stroustrup (of course), Bryce Adelstein Lelbach (library evolution subgroup chair), Hana Dusíková (compile-time programming subgroup chair), Inbal Levi (Israel national chair), JC Van Winkel (Netherlands national chair and teaching subgroup chair), JF Bastien (language evolution subgroup chair), Michael Wong (low-latency/gaming/embedded subgroup chair and AI subgroup chair), and Tony Van Eerd (expert in many subgroups and popular speaker). I can’t wait!
  • Wednesday 0730 MDT: Bjarne Stroustrup AMA (moderator). This is Bjarne’s AMA, I’m just the moderator.
  • (will be recorded) Friday 1330 MDT: Empirically Measuring, and Reducing, C++’s Accidental Complexity. This is my one actual talk, and it’s the last talk of the conference. It will be a major update of the talk I’ve given publicly one time before in Prague earlier this year, which after a broader intro focused specifically on parameter passing as an example of where we could dramatically simplify C++. This time I’ll include lots of updates, including that I hope to demo a working compiler implementation of the proposal.

Notes:

  • Denver time zone (MDT) is the default, which is where the physical CppCon usually happens. The Sched.org view lets you show the schedule in your own time zone.
  • The conference starts Sunday. You don’t have to wait for Bjarne’s opening keynote on Monday… if you’re attending, be sure to make use of the Open House from 0900-1200 MDT where you can wander around, and especially the Welcome Reception from 1200-1430 MDT which includes the first panel (see above).

I look forward to seeing many of you there!