Construct returned object in calling function's scope

Question

Is it possible to force C++ to construct an object in the scope of a calling function? What I mean is to explicitly do what an return value optimization (RVO) does.

I have some container classes which are in a chain of derivation. Since the classes are constructed with stack data, they can't be returned, so I disabled the copy constructor and assignment operators. For each class, I am providing an iterator. The constructor of each iterator has only one argument: a pointer to the container class. To get the iterator, I want to use this function:

BindPackIterator BindPack.begin(void)
{
    return BindPackIterator(this);
}

in this context:

for (auto i=bindpack.begin(); !i.end(); ++i) { i.run(); }

The compiler issues errors, complaining about not being able to copy the BindPackIterator object. Remember, I disabled them.

What I want to happen is for the BindPackIterator to be instantiated in the calling function's scope to avoid either a copy or move operation.

In this particular case, I know I can do a workaround, changing the begin function to return a BindPack pointer,

for(BindPackIterator i=bindpack.begin(); !i.end(); ++i) { i.run(); }

and I've experimented a bit, without success, with decltype and this construction:

auto BindPack::begin(void) -> BindPackIterator
{
    return BindPackIterator(this);
}

This is just the example with which I'm currently frustrated. There have been other projects where the obvious solution is for the function to instantiate an object in the calling function's scope. The move constructor (foo&&) helps in some cases, but for objects with many data members, even that can be inefficient. Is there a design pattern that allows object construction/instantiation in the caller's scope?

Answer 1

Putting n.m.'s comment into code, write a constructor for BindPackIterator that takes a BindPack and initializes the iterator in the "begin" state. e.g:

BindPackIterator(BindPack* pack) : pack(pack), pos(0){ }

That you can use in your for loop:

BindPack pack;

for(BindPackIterator i(&pack); !i.end(); ++i){
  i.run();
}

Live demo