C++ friend class std::vector

Question

Is it possible to do the following portably:

struct structure {
    structure() {}
private:
    // only allow container copy construct
    structure(const st         


        

Answer 1

No. vector (more precisely, the allocator passed into vector) can delegate the task of construction to a free function or another class, making the friendship useless.

Even if you pass your own allocator, it may be rebound to a class internal to the implementation. Then the constructor for your class may be accessed from that class, not your allocator. So if that's your workaround, it's not guaranteed. (Although looking at GCC's implementation, it does scrupulously use the un-rebound allocator to construct such subobjects.)

In GCC's libstdc++, no STL container template constructs the contained objects in the scope of a standard class or function.