René Richter René Richter - 17 days ago 7
C++ Question

Template parameter deduction for constructors from initializer list

At the Kona meeting, template parameter deduction for constructors (P0091R0) has been approved. It simplifies some variable definitions:

std::pair p {1,2}; // o.k., constructor pair<int,int>(1,2)
std::vector v1 (10, 0); // o.k., 10 zeroes, constructor vector<int>(size_t n, T initvalue)
std::vector v2 {10, 0}; // o.k., 2 values: 10, 0, apparently initializer list?
std::vector v3 = {10, 0}; // o.k., same as v2?


However, the following lines do not compile in gcc 7 HEAD 201611 version (live example):

std::vector v4 = {3}; // error: no matching function for call to 'std::vector(int)'
std::vector v5 {1, 2, 3}; // error: 'int' is not a class
std::set s {1, 2, 3}; // error: no matching function for call to 'std::set(int,int,int)'


Are these just "a bridge too far", since they involve initializer lists?
Are they covered by template type parameter deduction?
Will they be allowed, when compilers conform to C++1z?

Answer

You need an extra pair of curly braces to make your code work correctly:

std::vector v4 = {{1, 5}}; 
std::vector v5 {{1, 2, 3}};  
std::set    s  {{1, 2, 3}};

This is required due to rules concerning aggregate initialization: this question has more information on the matter.