Move constructors and inheritance

Question

I am trying to understand the way move constructors and assignment ops work in C++11 but I\'m having problems with delegating to parent classes.

The code:

Answer 1

You're only ever calling your base class's stuff with lvalues:

void foo(int&){}  // A
void foo(int&&){} // B

void example(int&& x)
{
    // while the caller had to use an rvalue expression to pass a value for x,
    // since x now has a name in here it's an lvalue:
    foo(x); // calls variant A
}

example(std::move(myinteger)); // rvalue for us, lvalue for example

That is, you need:

T(T&& o):
T0(std::move(o)) // rvalue derived converts to rvalue base
{
    puts("move");
}

And:

T& operator=(T&& o)
{
    puts("move assign");

    T0::operator=(std::move(o)));

    return *this;
}

Answer 2

One of the more confusing things about functions taking rvalue references as parameters is that internally they treat their parameters as lvalues. This is to prevent you from moving the parameter before you mean to, but it takes some getting used to. In order to actually move the parameter, you have to call std::move (or std::forward) on it. So you need to define your move constructor as:

T(T&& o): T0(std::move(o)) { puts("move"); }

and your move assignment operator as:

T& operator=(T&& o) { puts("move assign"); return static_cast<T&>(T0::operator=(std::move(o))); }