c++ temporary - "pure virtual method called"

Question

As I understand temporaries, the following code should work, but it doesn't.

struct base
{
    virtual~base() {}
    virtual void virt()const=0;
};
struct derived:public base
{
    virtual void virt()const {}
};

const base& foo() {return derived();}

int main()
{
    foo().virt();
    return 0;
}

The call to virt() gives a "pure virtual function called" error. Why is that, and what should I do?

Answer 1

You're returning a reference to a temporary which is destructed when the function ends at the end of the return statement and you get undefined behaviour.

You cannot return any kind of reference to a temporary and if you return a base by value you'll get slicing, so if you really want this to work, you should return a std::unique_ptr<base>.