struct initialization with in-class member initializers - c++

Question

I'm having trouble initializing a struct that uses in-class initializers:
struct A
{
int a{};
int b{};
};
struct B
{
int a;
int b;
};
int main()
{
A a; // OK
B b{1, 2}; // OK
B b2; // OK, but b.a and b.b are undefined
A a2{1, 2}; // ERROR!
}
Here's the error I'm getting from gcc 4.7.2:
% g++ -std=c++11 test2.cc
test2.cc: In function ‘int main()’:
test2.cc:16:11: error: no matching function for call to ‘A::A(<brace-enclosed initializer list>)’
test2.cc:16:11: note: candidates are:
test2.cc:1:8: note: constexpr A::A()
test2.cc:1:8: note: candidate expects 0 arguments, 2 provided
test2.cc:1:8: note: constexpr A::A(const A&)
test2.cc:1:8: note: candidate expects 1 argument, 2 provided
test2.cc:1:8: note: constexpr A::A(A&&)
test2.cc:1:8: note: candidate expects 1 argument, 2 provided
Should this work according to the standard, or is this actually illegal? Am I abusing the use of in-class initializers? I thought the new syntax would make it so I wouldn't have to write a constructor just to do this initialization, but now it seems that I may have to resort to that old mechanism to avoid the possibility of an uninitialized structure.

Answer 1

You can only use braces if either
the brace content matches a constructor (not your case), or
the class is an aggregate and each brace element matches a class member.
However, a class is an aggregate if (C++11, 8.5.1/1):
it has no brace-or-equal-initializers for non-static members
which your class clearly has. So, you don't have an aggregate, either.
Either write suitable constructors, or remove the brace-or-equal-initializers.