What Is a Curly-Brace Enclosed List If Not an intializer_list?

Question

I asked a question here: Lifetime Extension of a initializer_list return involving the non-functional code:

const auto foo = [](const auto& a, const auto& b, const auto& c) { return {a, b, c}; }; 

I believed the lambda was trying to return an intializer_list (that's bad, don't do that.) But I got a comment:

It's not an initializer_list, it's an initializer list. Two different things.

I just thought that any time you did a curly-braced list you were creating an intializer_list. If that's not what's happening, what is a list in curly-braces?

Answer 1

It is an braced-init-list. A braced-init-list existed before std::initializer_list and is used to initialize aggregates.

int arr[] = {1,2,3,4,5}; 

The above used a braced-init-list to initialize the array, no std::initializer_list is created. On the other hand when you do

std::vector<int> foo = {1,2,3,4,5}; 

foo is not an aggregate so the braced-init-list is used to create a std::initializer_list which is in turned passed to the constructor of foo that accepts a std::initializer_list.

A thing to note about a braced-init-list is that is has no type so special rules were developed for use with it and auto. It has the following behavior (since the adoption of N3922)

auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int> auto x2 = { 1, 2.0 }; // error: cannot deduce element type auto x3{ 1, 2 }; // error: not a single element auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int> auto x5{ 3 }; // decltype(x5) is int 

And you can get more information on history of this behavior and why it was changed at: Why does auto x{3} deduce an initializer_list?

Answer 2

There are three distinct, but related concepts here:

1. braced-init-list: The grammatical rule associated with curly-brace-enclosed lists in certain contexts.

2. Initializer list: The name for the braced-init-list initializer used in list-initialization.

3. std::initializer_list: A class wrapping a temporary array which is created in some contexts involving braced-init-lists.

Some examples:

//a braced-init-list and initializer list,  //but doesn't create a std::initializer_list int a {4};   //a braced-init-list and initializer list, //creates a std::initializer_list std::vector b {1, 2, 3};  //a braced-init-list and initializer list, //does not create a std::initializer_list (aggregate initialization) int c[] = {1, 2, 3};  //d is a std::initializer_list created from an initializer list std::initializer_list d {1, 2, 3};  //e is std::initializer_list<int> auto e = { 4 };  //f used to be a std::initializer_list<int>, but is now int after N3922 auto f { 4 }; 

You might want to read N3922, which changed some of the rules involving auto and std::initializer_list.