Making swap faster, easier to use and exception-safe

Question

I could not sleep last night and started thinking about std::swap. Here is the familiar C++98 version:

template 
void swap(T&a         


        

Answer 1

your swap version will cause havoc if someone uses it with polymorphic types.

consider:

Base *b_ptr = new Base();    // Base and Derived contain definitions
Base *d_ptr = new Derived(); // of a virtual function called vfunc()
yourmemcpyswap( *b_ptr, *d_ptr );
b_ptr->vfunc(); //now calls Derived::vfunc, while it should call Base::vfunc
d_ptr->vfunc(); //now calls Base::vfunc while it should call Derived::vfunc
//...

this is wrong, because now b contains the vtable of the Derived type, so Derived::vfunc is invoked on a object which isnt of type Derived.

The normal std::swap only swaps the data members of Base, so this is OK with std::swap