C++17 New Rules For auto Deduction From braced-init-list

Initialization of variables in C++ can have several forms:

• default initialization: std::string s;
• value initialization: std::string s{};
• direct initialization: std::string s("demo");
• copy initialization: std::string s = "demo";
• list initialization: std::string s{'d', 'e', 'm', 'o'};
• aggregate initialization: char s[5] = {'d', 'e', 'm', 'o'};
• reference initialization: char& c = s[0];

C++11 introduced a generalized syntax for initialization with a braced initializer list, referred to as braced-init-list. Initialization with braced-init-list is called list initialization. There are two types of list initialization, each having multiple forms (check the above links), but simplified, we can have:

• direct list initialization: T object {arg1, arg2, ...};
• copy list initialization: T object = {arg1, arg2, ...};

Prior to C++17 the type for all the following objects (a, b, c and d) is deduced to std::initializer_list<int>. There is no difference between the direct-list-initialization and the copy-list-initialization on the result of the type deduction.

auto a = {42};   // std::initializer_list<int>
auto b {42};     // std::initializer_list<int>
auto c = {1, 2}; // std::initializer_list<int>
auto d {1, 2};   // std::initializer_list<int>

This, however, changed in C++17 that introduced the following rules:

• for copy list initialization auto deduction will deduce a std::initializer_list<T> if all elements in the list have the same type, or be ill-formed.
• for direct list initialization auto deduction will deduce a T if the list has a single element, or be ill-formed if there is more than one element.

As a result, the example above changes so that a and c are still std::initializer_list<int> but b is deduced as an int and d is ill-formed, because there is more than one value in the brace-init-list.

auto a = {42};   // std::initializer_list<int>
auto b {42};     // int
auto c = {1, 2}; // std::initializer_list<int>
auto d {1, 2};   // error, too many 

For more information about these changes see N3922: New Rules for auto deduction from braced-init-list.