Using std::array with initialization lists

Question

Unless I am mistaken, it should be possible to create a std:array in these ways:

std::array<std::string, 2> strings = { "a", "b" };
std::array<std::string, 2> strings({ "a", "b" });

And yet, using GCC 4.6.1 I am unable to get any of these to work. The compiler simply says:

expected primary-expression before ',' token

and yet initialization lists work just fine with std::vector. So which is it? Am I mistaken to think std::array should accept initialization lists, or has the GNU Standard C++ Library team goofed?

Answer 1

std::array is funny. It is defined basically like this:

template<typename T, int size>
struct std::array
{
  T a[size];
};

It is a struct which contains an array. It does not have a constructor that takes an initializer list. But std::array is an aggregate by the rules of C++11, and therefore it can be created by aggregate initialization. To aggregate initialize the array inside the struct, you need a second set of curly braces:

std::array<std::string, 2> strings = {{ "a", "b" }};

Note that the standard does suggest that the extra braces can be elided in this case. So it likely is a GCC bug.

Answer 2

To add to the accepted answer:

std::array<char, 2> a1{'a', 'b'};
std::array<char, 2> a2 = {'a', 'b'};
std::array<char, 2> a3{{'a', 'b'}};
std::array<char, 2> a4 = {{'a', 'b'}};

all work on GCC 4.6.3 (Xubuntu 12.01). However,

void f(std::array<char, 2> a)
{
}

//f({'a', 'b'}); //doesn't compile
f({{'a', 'b'}});

the above requires double braces to compile. The version with single braces results in the following error:

../src/main.cc: In function ‘int main(int, char**)’:
../src/main.cc:23:17: error: could not convert ‘{'a', 'b'}’ from ‘<brace-enclosed initializer list>’ to ‘std::array<char, 2ul>’

I'm not sure what aspect of type inference/conversion makes things work this way, or if this is a quirk of GCC's implementation.