array member initialization of user defined types

Question

g++ 4.7 supports array member initialization and I started playing with it.

The code below does not compile.

struct A
{
   A(int){};
   A(const A&) = delete;
   A& operator=(const A&) = delete;
   ~A(){};
};

struct B
{
   B():
      a{{0},{1}}
   {};
   A a[2];
};

B b;

The error message with gcc 4.8 (prerelease) is:

n.cc: In constructor ‘B::B()’:
n.cc:12:20: error: use of deleted function ‘A::A(const A&)’
           a{{0},{1}}
             ^
n.cc:4:8: error: declared here
        A(const A&) = delete;
        ^

Is there a way to make this code work? I can't easily change the contructors,destructor of A. I seem to need a move-constructor or copy-constructor to initialize the array, but this seems counter-intuitive, since all I really want is in-place construction.

It works if I split a[2] in 2 members a0 and a1, and construct them separately. This looks fishy however.

Answer 1

Within list-initialization (8.5.4p1) of an aggregate (8.5.1), the form of initialization performed on the elements of the aggregate is copy-initialization (8.5.1p2), even if the initialization is direct-list-initialization:

When an aggregate is initialized by an initializer list, as specified in 8.5.4, the elements of the initializer list are taken as initializers for the members of the aggregate, in increasing subscript or member order. Each member is copy-initialized from the corresponding initializer-clause.

However, just because the form of initialization performed is copy-initialization doesn't mean that a copy occurs. As per copy-list-initialization of non-copyable types, copy-list-initialization should be identical to direct-list-initialization with the exception that explicit constructors are not allowed.