Question

I have this code in a library:

class Parent
{
    //some data and functions
};

void myfunc(Parent& ref);

and I want to do this code in my application:

class Child : public Parent
{
   // some other data and functions
   void dostuff()
   {
       myfunc(*this);
   }
};

Is it safe to pass *this? (no slicing, no copying, …)
Is it better to call myfunc like this:

myfunc( * ((Parent*)this) )

Note that I don’t have control on what happens inside myfunc, in some cases I don’t even know what happens inside there.

I used passing-parent-by-pointer many times and I am used to it, but have never used passing-parent-by-reference before.

Answer 1

myfunc(*this) is fine, so long as myfunc is declared to take a reference — which it is.

This will not copy the object. It will pass a reference to the original object. Furthermore, it will not slice the object. The reference will be of type Base&, but the object to which it refers will be unchanged.

Just so you know, if you were then to call polymorphic (eg, virtual) methods on this Base&, polymorphism will still work right and do what you’d expect — just like if you were to call through a pointer. In other words:

Base& b = *derived;
b.SomeVirtualFoo();

…will have the same effect as:

Base* b = derived;
b->SomeVirtualFoo();