GotW #100 Solution: Preconditions, Part 1 (Difficulty: 8/10)

This special Guru of the Week series focuses on contracts. We’ve seen how postconditions are directly related to assertions (see GotWs #97 and #99). So are preconditions, but that in one important way makes them fundamentally different. What is that? And why would having language support benefit us even more for writing preconditions more than for the other two?

1. What is a precondition, and how is it related to an assertion?

A precondition is a “call site prerequisite on the inputs”: a condition that must be true at each call site before the caller can invoke this function. In math terms, it’s about expressing the domain of recognized inputs for the function. If preconditions don’t hold, the function can’t possibly do its work (achieve its postconditions), because the caller hasn’t given it a starting point it understands.

A precondition IS-AN assertion in every way described in GotW #97, with the special addition that whereas a general assertion is always checked where it is written, a precondition is written on the function and conceptually checked at every call site. (In less-ideal implementations, including if we write it as a library today, the precondition check might be in the function body; see Question 2.)

Explain your answer using the following example, which uses a variation of a proposed post-C++20 syntax for preconditions. [1]

// Example 1(a): A precondition along the lines proposed in [1]

void f( int min, int max )
    [[pre( min <= max )]]
    // ...

The above would be roughly equivalent to writing the test before the call at every call site instead. For example, for a call site that performs f(x, y), we want to check the precondition at this specific call site at least when it is being tested (and possibly earlier and/or later, see GotW #97 Question 4):

// Example 1(b): What a compiler might generate at a call site
//               “f(x, y)” for the precondition in Example 1(a)

assert( x <= y ); // implicitly injected assertion at this call site,
                  // checked (at least) when this call site is tested
f(x, y);

And, as we’ll see in Question 4, language support for preconditions should apply this rewrite recursively for subexpressions that are themselves function calls with preconditions.

GUIDELINE: Use a precondition to write “this is what a bug is” as code the caller can check. A precondition states in code the circumstances under which this function’s behavior is not documented.

2. Rewrite the example in Question 1 to show how to approximate the same effect using assertions in today’s C++.

Here’s one way we can do it, that extends the MY_POST technique from GotW #99 Example 2 to also support preconditions. Again, instead of MY_ you’d use your company’s preferred unique macro prefix: [2]

// Eliminate forward-boilerplate with a macro (written only once)
#define MY_PRE_POST(preconditions, postconditions)         \
    assert( preconditions );                               \
    auto post = [&](auto&& _return_) -> auto&& {           \
        assert( postconditions );                          \
        return std::forward<decltype(_return_)>(_return_); \

And then the programmer can just write:

// Example 2: Sample precondition

void f( int min, int max )
{   MY_PRE_POST_V( min <= max, true ); // true == no postconditions here
    // ...

This has the big benefit that it works using today’s C++. It has the same advantages as MY_POST in GotW #99, including that it’s future-friendly… if we use the macro as shown above, then if in the future C++ has language support for preconditions and postconditions with a syntax like [1], migrating your code to that could be as simple as search-and-replace:

{ MY_PRE_POST( **, * )[[pre: ** ]] [[post _return_: * )]] {

return post( * )return *

GUIDELINE (extended from GotW #99): If you don’t already use a way to write preconditions and postconditions as code, consider trying something like MY_PRE_POST until language support is available. It’s legal C++ today, it’s not terrible, and it’s future-friendly to adopting future C++ language contracts.

But even if macros don’t trigger your fight-or-flight response, it’s still a far cry from language support …

Are there any drawbacks to your solution compared to having language support for preconditions?


  • Callee-body checking only. This method can run the check only inside the function’s body. First, this means we can’t easily perform the check at each call site, which would be ideal including so we can turn the check on for one call site but not another when we are testing a specific caller. Second, for constructors it can’t run at the very beginning of construction because member initialization happens before we enter the constructor body.
  • Doesn’t directly handle nested preconditions, meaning preconditions of functions invoked as part of the precondition itself. We’ll come to this in Question 4.

3. If a precondition fails, what does that indicate, and who is responsible for fixing the failure?

Each call site is responsible for making sure it meets all of a function’s preconditions before calling that function. If a precondition is false, it’s a bug in the calling code, and it’s the calling code author who is responsible for fixing it.

Explain how this makes a precondition fundamentally different from every other kind of contract.

A precondition is the only kind of contract you can write that someone else has to fulfill, and so if it’s ever false then it’s someone else’s fault — it’s the caller’s bug that they need to go fix.

GUIDELINE: Remember the fundamental way preconditions are unique… if they’re false, then it’s someone else’s fault (the calling code author). When you write any of the other contracts (assertions, function postconditions, class invariants), you state something that must be true about your own function or class, and if prior contracts were written and well tested then likely it’s your function or class that created the first unexpected state.

4. Consider this example, expanded from a suggestion by Gábor Horváth:

// Example 4(a): What are the implicit preconditions?

auto calc( std::vector<int> const&  x ,
           std::floating_point auto y ) -> double
    [[pre( x[0] <= std::sqrt(y) )]] ;

Note that std::floating_point is a C++20 concept.

a) What kinds of preconditions must a caller of calc satisfy that can’t generally be written as testable boolean expressions?

The language requires the number and types of arguments to match the parameter list. Here, calc must be called with two arguments. The first must be a std::vector<int> or something convertible to that. The second one’s type has to satisfy the floating_point concept (it must be float, double, or long double).

It’s worth remembering that these language-enforced rules are conceptually part of the function’s precondition, in the sense that they are requirements on call sites. Even though we generally can’t write testable boolean predicates for these to check that we didn’t write a bug, we also never need to do that because if we write a bug the code just won’t compile. [3] Code that is “correct by construction” doesn’t need to add assertions to find potential bugs.

GUIDELINE: Remember that a static type is a (non-boolean) precondition. It’s just enforced by language semantics with always-static checking (edit or compile time), and never needs to be tested using a boolean predicate whose test could be delayed until dynamic checking (test or run time).

COROLLARY: A function’s number, order, and types of parameters are all (non-boolean) parts of its precondition. This falls out of the “static type” statement because the function’s own static type includes those things. For example, the language won’t let us invoke this function with the argument lists () or (1,2,3,4,5) or (3.14, myvector). We’ll delve into this more deeply in GotW #101.

COROLLARY: All functions have preconditions. Even void f() { }, which takes no inputs at all including that it reads no global state, has the precondition that it must be passed zero arguments. The only counterexample I can think of is pathological: void f(...) { } can be invoked with any number of arguments but ignores them all.

b) What kinds of boolean-testable preconditions are implicit within the explicitly written declaration of calc?

There are three possible kinds of implied boolean preconditions. All three are present in this example.

(1) Type invariants

Each object must meet the invariants of its type. This is subtly different from “the object’s type matches” (a static property) that we say in 4(a), because this means additionally “the object’s value is not corrupt” (a dynamic property).

Here, this means x must obey the invariant of vector<int>, even though that invariant isn’t expressed in code in today’s C++. [4] For y this is fairly easy because all bit patterns are valid floating point values (more about NaNs in just a moment).

(2) Subexpression preconditions

The subexpression x[0] calls x.operator[] which has its own precondition, namely that the subscript be non-negative and less than x.size(). For 0, that’s true if x.size() > 0 is true, or equivalently !x.empty(), so that becomes an implicit part of our whole precondition.

(3) Subexpressions that make the whole precondition false

The subexpression std::sqrt(y) invokes C’s sqrt. The C standard says that unless y >= 0, the result of sqrt(y) is NaN (“not a number”), which means our precondition amounts to something <= NaN which is always false. Therefore, y >= 0 is effectively part of calc’s precondition too. [5]

Putting it all together

If we were to write this all out, the full precondition would be something like this — and note that the order is important! Here we’ll ignore the parts that are enforced by the language, such as parameter arity, and focus on the parts that can be written as boolean expressions:

// Example 4(b): Trying to write the precondition out more explicitly
//               (NOT all are recommended, this is for exposition)

auto calc( std::vector<int> const&  x ,
           std::floating_point auto y ) -> double

//  1. parameter type invariants:
           /* x is a valid object, but we can’t spell that, so: */ true

//  2. subexpression preconditions:
        && x.size() > 0  // so checking x[0] won’t be undefined (!)

//  3. subexpression values that make our precondition false:
        && y >= 0        // redundant with the expression below

// finally, our explicit precondition itself:
        && x[0] <= std::sqrt(y) 
    )]] ;

GUIDELINE: Remember that your function’s full effective precondition is the precondition you write plus all its implicit prerequisites. Those are: (1) each parameter’s type invariants, (2) any preconditions of other function calls within the precondition, and (3) any defined results of function calls within the precondition that would make the precondition false.

c) Should any of these boolean-testable implicit preconditions also be written explicitly here in this precondition code? Explain.

For #1 and #3, we generally shouldn’t be repeating them as in 4(b):

  • We can skip repeating #1 because it’s enforced by the type system, plus if there is a bug it’s likely in the type itself rather than in our code or our caller’s code and will be checked when we check the type’s invariants.
  • We can skip repeating #3 because it’ll just make the whole condition be false and so is already covered.

But #2 is the problematic case: If x is actually empty, the subexpression’s precondition would actually make our precondition undefined to evaluate! “Undefined” is a very bad answer if we ever check this precondition, because if in our checking the full precondition is ever violated then we absolutely want that check to do something well-defined — we want it to evaluate to false and fail.

If a subexpression of our precondition itself has a real precondition, then we do want to check that first, otherwise we cannot check our full precondition without undefined behavior if that subexpression’s precondition was not met:

// Example 4(c): Today, we should repeat our subexpressions’ real
//               preconditions, so we can check our precondition
//               without undefined behavior

auto calc( std::vector<int> const&  x ,
           std::floating_point auto y ) -> double
    [[pre( x.size() > 0 && x[0] <= std::sqrt(y) )]] ;

With today’s library-based preconditions, such as the one shown in Question 2, we need to repeat subexpressions’ preconditions if we want to check our precondition without undefined behavior. One of the potential advantages of a language-supported contracts system is that it can “flatten” the preconditions to automatically test category #2 , so that nested preconditions like this one don’t need to be repeated (assuming that the types and functions you use, here std::vector and its member functions, have written their preconditions and invariants)… and then we could still debate whether or not to explicitly repeat subexpression preconditions in our preconditions, but it would be just a water-cooler stylistic debate, not a “can this even be checked at all without invoking undefined behavior” correctness debate.

Here’s a subtle variation suggested by Andrzej Krzemieński. For the sake of discussion, suppose we have a nested precondition that is not used in the function body (which I think is terribly unlikely, but let’s just consider it):

void display( /*...*/ )
    [[pre( globalData->helloMessageHasBeenPrinted() )]]
    // assume for sake of discussion that globalData is not
    // dereferenced directly or indirectly by this function body

Here, someone could argue: “If globalData is null, only actually checking the precondition would be undefined behavior, but executing the function body would not be undefined behavior.”

Question: Is globalData != nullptr an implicit precondition of display, since it applies only to the precondition, and is not actually used in the function body? Think about it for a moment before continuing…

… okay, here’s my answer: Yes, it’s absolutely part of the precondition of display, because by definition a precondition is something the caller is required to ensure is true before calling display, and a condition that is undefined to evaluate at all cannot be true.

GUIDELINE: If your checked precondition has a subexpression with its own preconditions, make sure those are checked first. Otherwise, you might find your precondition check doesn’t fire even when it’s violated. In the future, language support for preconditions might automate this for you; until then, be careful to write out the subexpression precondition by hand and put it first.


[1] 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.

[2] Again, as in GotW #99 Note 4, in a real system we’d want a few more variations, such as:

// A separate _V version for functions that don’t return
// a value, because 'void' isn’t regular
#define MY_PRE_POST_V(preconditions, postconditions) \
    assert( preconditions );                         \
    auto post = [&]{ assert( postconditions ); };

// Parallel _DECL forms to work on forward declarations,
// for people who want to repeat the postcondition there
#define MY_PRE_POST_DECL(preconditions, postconditions)
#define MY_PRE_POST_V_DECL(preconditions, postconditions)

And see GotW #99 Note 5 for how to guarantee the programmer didn’t forget to write “return post” at each return.

[3] Sure, there are things like is_invocable, but the point is we can’t always write those expressions, and we don’t have to here.

[4] Upcoming GotWs will cover invariants and violation handling. For type invariants, today’s C++ doesn’t yet provide a way to write those as a checkable assertions to help us find bugs where we got it wrong and corrupted an object. The language just flatly assumes that every object meets the invariants of its type during the object’s lifetime, which is from the end of its construction to the beginning of its destruction.

[5] There’s more nuance to the details of what the C standard says, but it ends up that we should expect the result of passing a negative or NaN value to sqrt will be NaN. Although C calls negative and NaN inputs “domain errors,” which hints at a precondition, it still defines the results for all inputs and so strictly speaking doesn’t have a precondition.


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