c++ - What's wrong with this inline initialization of std::array?

Question

Consider the following declaration:

#include <array>

struct X
{
    //std::array<bool,3> arr={false,false,false};
    bool brr[3]={false,false,false};
};

As is, it compiles normally by g++ 5.2. But if I uncomment the std::array, I get an error:

test.cpp:5:46: error: array must be initialized with a brace-enclosed initializer
     std::array<bool,3> arr={false,false,false};
                                              ^
test.cpp:5:46: error: too many initializers for ‘std::array<bool, 3u>’

OTOH, this declaration works without problems inside main(). Also, the following initialization does work inside struct X:

std::array<bool,3> arr={{false,false,false}};

Why can't I use the simple initialization with single braces in struct definition?

Answer 1

This looks like a gcc bug see: Bug 65815 - brace elision doesn't work in NSDMI. The report says:

On Page 975 of "The C++ Programming Language", 4th edition, Bjarne Stroustrup says:

"An array can be initialized by an initializer list: array a1 = { 1, 2, 3 };"

and Clang (V 3.5) accepts it. However, G++ 4.9.2 thinks this is an error:

"error: array must be initialized with a brace-enclosed initializer
   const std::array<double, 3> _ar0val = {1.0, -1.0, 1.0};"

The issue was narrowed down to the following test case:

struct array {
  int data [2];
};

struct X {
  array a = { 1, 2 };
};

It looks like the fix is in the head revision, the OPs code works in that revision, see it live.

As noted in the bug report using an inner set of braces is a possible work-around:

std::array<bool,3> arr={ {false,false,false} };
                         ^                 ^