Differentiating Prefix and Postfix Operators

There is one problem with defining both the prefix and postfix operators: Normal overloading cannot distinguish between these operators. The prefix and postfix versions use the same symbol, meaning that the overloaded versions of these operators have the same name. They also have the same number and type of operands.

To solve this problem, the postfix versions take an extra (unused) parameter of type int. When we use a postfix operator, the compiler supplies 0 as the argument for this parameter. Although the postfix function can use this extra parameter, it usually should not. That parameter is not needed for the work normally performed by a postfix operator. Its sole purpose is to distinguish a postfix function from the prefix version.

We can now add the postfix operators to StrBlobPtr:

class StrBlobPtr {
public:
    // increment and decrement
    StrBlobPtr operator++(int);    // postfix operators
    StrBlobPtr operator--(int);
    // other members as before
};

The postfix versions have to remember the current state of the object before incrementing the object:

// postfix: increment/decrement the object but return the unchanged value
StrBlobPtr StrBlobPtr::operator++(int)
{
    // no check needed here; the call to prefix increment will do the check
    StrBlobPtr ret = *this;   // save the current value
    ++*this;     // advance one element; prefix ++ checks the increment
    return ret;  // return the saved state
}
StrBlobPtr StrBlobPtr::operator--(int)
{
    // no check needed here; the call to prefix decrement will do the check
    StrBlobPtr ret = *this;  // save the current value
    --*this;     // move backward one element; prefix -- checks the decrement
    return ret;  // return the saved state
}

Each of our operators calls its own prefix version to do the actual work. For example, the postfix increment operator executes

++*this

This expression calls the prefix increment operator. That operator checks that the increment is safe and either throws an exception or increments curr. Assuming check doesn’t throw an exception, the postfix functions return the stored copy in ret. Thus, after the return, the object itself has been advanced, but the value returned reflects the original, unincremented value.