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

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.