Question

I recently reworked one of my own libraries to try out separating interface from implementation. I am having on final issue with a class that is meant to return an instance of another class.

In the interface definition, I do something like

struct IFoo
{
    virtual const IBar& getBar() = 0;
}

and then in the concrete Foo getBar looks like

const IBar& Foo::getBar()
{
    Bar ret = Bar();
    return ret;
}

The problem is ret gets deleted as soon as getBar is done, causing a big crash when the copy constructor tries to use Bar like so

const Bar myBar = myFoo.getBar();

I have been reading various things, and I know returning by reference is frowned upon, but I do not see any other way (I do not want to return Bar* because I do not want to have to manually delete the return value).

What is the proper way (if any way exists) for an abstract class to return an instance of a concrete class derived from another abstract class?

Note I did see this solution: returning an abstract class from a function
but I do not want to make the return value static and loose thread safety.

Answer 1

Use smart pointers.
These are pointers deleted when not used anymore (see for example http://www.boost.org/doc/libs/1_43_0/libs/smart_ptr/smart_ptr.htm).