I think this answer might be relevant.
Yes, this behaviour is intended, according to §13.3.1.7 Initialization by list-initialization
When objects of non-aggregate class type T are list-initialized (8.5.4), overload resolution selects the constructor in two phases:
— Initially, the candidate functions are the initializer-list constructors (8.5.4) of the class T and the argument list consists of the initializer list as a single argument.
— If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.
In gcc
I tried your example. I get this error:
error: call of overloaded 'A(<brace-enclosed initializer list>)' is ambiguous
gcc
stops complaining if I use three sets of brace. i.e.:
#include <iostream>
#include <vector>
#include <initializer_list>
struct A {
A (std::initializer_list<int> il) {
std::cout << "First." << std::endl;
}
A (std::initializer_list<std::initializer_list<int>> il) {
std::cout << "Second." << std::endl;
}
};
int main()
{
A a{0}; // first
A a{{0}}; // compile error
A a2{{{0}}}; // second
A a3{{{{0}}}}; // second
}
In an attempt to mirror the vector's constructors, here are my results:
#include <iostream>
#include <vector>
#include <initializer_list>
struct A {
A (std::initializer_list<int> il) {
std::cout << "First." << std::endl;
}
explicit A (std::size_t n) {
std::cout << "Second." << std::endl;
}
A (std::size_t n, const int& val) {
std::cout << "Third." << std::endl;
}
A (const A& x) {
std::cout << "Fourth." << std::endl;
}
};
int main()
{
A a{0};
A a2{{0}};
A a3{1,2,3,4};
A a4{{1,2,3,4}};
A a5({1,2,3,4});
A a6(0);
A a7(0, 1);
A a8{0, 1};
}
main.cpp:23:10: warning: braces around scalar initializer
A a2{{0}};
^~~
1 warning generated.
First.
First.
First.
First.
First.
Second.
Third.
First.