The Bristol meeting concluded a few hours ago, and I just posted my trip report on isocpp.org:

This afternoon in Bristol, UK, the ISO C++ standards committee adopted generic lambdas, dynamic arrays (an improved version of C99 VLAs), variable templates, reader/writer locks, make_unique, optional<T>, standard library user-defined literals, and a number of other language and library improvements – and approved the result as the feature-complete Committee Draft (CD) of Standard C++14 to be distributed for its primary international review ballot.

In addition to completing the C++14 CD document, the committee also made progress on three additional important parallel specifications that are on track to be published around the same time as C++14:

• File system library (draft), based on Boost.FileSystem version 3.
• Networking library, small at first and regularly extended.
• “Concepts Lite” language extensions (draft), to express template constraints and improve template usability and error messages.

Together these mark the C++ committee’s main planned deliverables for 2014. …

To provide just a sampling [of C++14], here are a few quick examples of some of the newly added features…

Continue reading…

On std-discussion, Shakti Misra asked:

I have seen in a lot of places code like
int i;
if(someConditionIstrue)
{
Do some operations and calculate the value of i;
i = some calculated value;
}
use i; //Note this value is only used not changed. It should not be changed.
But unfortunately in this case there is no way to guarantee it.
so now if some one comes and does

i = 10;// This is valid

What i was thinking: is there a way to tell that "i" can be set only once?
After that it will be a constant. Something like

const once int i;
if(someConditionIstrue)
{
Do some operations and calculate the value of i;
i = calculated value;
}
use i; //Note this value is only used not changed.
i = 10;// Compiler error

Olaf van der Spek replied:

A lambda?

I answered as follows:

Bingo. Olaf nailed it: The way to do it is with a lambda. This is one of the examples I give in my talk Lambdas, Lambdas Everywhere (here are the slides from C++ & Beyond 2010).

In your example, do this:

 

const int i = [&]{

    int i = some_default_value;

    if(someConditionIstrue)
    {
        Do some operations and calculate the value of i;
        i = some calculated value;
    }

    return i;

} (); // note: () invokes the lambda!

 

That’s the C++11 idiom for this. It’s still being absorbed though – not everyone knows about it yet as we’re still all learning C++11 styles together.