Question

Consider:

struct A {

 A (int);

 A (const A &);
};

struct B {

 A foo [2];

 B (const A & x, const A & y)
 : foo {x, y} /* HERE IS THE PROBLEM */
 {}
};

I was expecting this to work since I’m using C++0x support in GCC4.3, which allegedly supports initialiser lists. No joy.

I have a class A which has no default constructor. This is not negotiable. Assignment post-default is not an option.

I am trying to create B which uses A. B::foo may not be std::vector.

How can I initialise B::foo in B(...), constructing its elements exactly once?

At the moment, I am condidering replacing B with

struct B {

A foo_a;

B foo_b;

A * foo () {
assert ((&foo_b) - *&foo_a) == 1);
return &foo_a;
}

B (const A & x, const A & y) : foo_a(x), foo_b(y) {}
};

Or even using char foo [2*sizeof(A)] with placement new — YUK!

Surely there’s a proper way to do this?

Answer 1

Unfortunately, there really is no proper, clean way to do this. Consider it something of a language limitation that results from an awkward mixing of C++ constructors and C style arrays. The C++11 standard addresses this issue, but until then you’ll have to settle for a workaround.

Since A has no default constructor, one possible work-around is to have an array of A* pointers, and then loop over the array and initialize each one with new. (Obviously, don’t forget to delete each item in the array in B’s destructor, or just use smart pointers.)