overloaded increment's return value

Question

In his The C++ Programming Language Stroustrup gives the following example for inc/dec overloading:

class Ptr_to_T {
 T* p;
 T* array ;
 int size;
public:
 Ptr_to_T(T* p, T* v, int s); // bind to array v of size s, initial value p
 Ptr_to_T(T* p); // bind to single object, initial value p
 Ptr_to_T& operator++(); // prefix
 Ptr_to_T operator++(int); // postfix
 Ptr_to_T& operator--(); // prefix
 Ptr_to_T operator--(int); // postfix
 T&operator*() ; // prefix
}

Why prefix operators return by reference while postfix operators return by value?

Thanks.

Answer 1

The postfix operator returns a copy of the value before it was incremented, so it pretty much has to return a temporary. The prefix operator does return the current value of the object, so it can return a reference to, well, its current value.

Answer 2

To understand better, you have to imagine (or look at) how are these operators implemented. Typically, the prefix operator++ will be written more or less like this:

MyType& operator++()
{
    // do the incrementation
    return *this;
}

Since this has been modified "in-place", we can return a reference to the instance in order to avoid a useless copy.

Now, here's the code for the postfix operator++:

MyType operator++(int)
{
    MyType tmp(*this); // create a copy of 'this'
    ++(*this); // use the prefix operator to perform the increment
    return tmp; // return the temporary
}

As the postfix operator returns a temporary, it has to return it by value (otherwise, you'll get a dangling reference).

The C++ Faq Lite also has a paragraph on the subject.

Answer 3

Suppose I use overloaded preincrement to increment a private member. Doesn't returning a reference to a private member turns the ++private_var expression to an lvalue thus making it possible to modify the private member directly?