Value cast vs Reference cast

Question

What is the difference between value cast and reference cast? Why one of them invokes conversion (aka creating new object) and other doesn't? What are caveats of using casting on rhs?

Assume this is Derived . Why those will not actually cast to Base?

*this = (Base&) rhs
(Base)* this = rhs

Could you please show on simple examples?

Answer 1

Value cast creates a new value from an existing one; reference cast creates a new reference to the same existing value.

Reference cast neither changes the content of an existing object nor creates a new one; it is restricted to changing the interpretation of the value that is already there. Value casting, on the other hand, can make a new object from an existing one, so it has fewer restrictions.

For example, if you have an unsigned char and you want a value or a reference of type int, value cast is going to work, while reference casting is going to fail:

unsigned char orig = 'x';
int  v(orig); // Works
int &r(orig); // Does not work

rhs is Derived, I want to assign all inherited and non-inherited stuff from rhs into Base

Then you need to cast both sides to Base, or add an assignment operator to Derived that takes a const Base& as an argument. Here is an example of casting on both sides (may be hard to understand by other programmers reading your code)

struct Base {
    int x;
    Base(int x) : x(x) {}
};
struct Derived1 : public Base {
    Derived1(int x) : Base(x) {}
};
struct Derived2 : public Base {
    Derived2(int x) : Base(x) {}
};

Running the code below

Derived1 d1(5);
Derived2 d2(10);
cout << d1.x << " " << d2.x << endl;
((Base&)d1) = (Base&)d2;
cout << d1.x << " " << d2.x << endl;

produces the following printout:

5 10
10 10

As you can see, the assignment ((Base&)d1) = (Base&)d2 copied the content of d2's Base portion into d1's Base portion (demo).