Combining Dereference and Increment in a Single Expression

The postfix versions of ++ and -- are used when we want to use the current value of a variable and increment it in a single compound expression.

As one example, we can use postfix increment to write a loop to print the values in a vector up to but not including the first negative value:

auto pbeg = v.begin();
// print elements up to the first negative value
while (pbeg != v.end() && *beg >= 0)
    cout << *pbeg++ << endl; // print the current value and advance pbeg

The expression *pbeg++ is usually confusing to programmers new to both C++ and C. However, because this usage pattern is so common, C++ programmers must understand such expressions.

The precedence of postfix increment is higher than that of the dereference operator, so *pbeg++ is equivalent to *(pbeg++). The subexpression pbeg++ increments pbeg and yields a copy of the previous value of pbeg as its result. Accordingly, the operand of * is the unincremented value of pbeg. Thus, the statement prints the element to which pbeg originally pointed and increments pbeg.

This usage relies on the fact that postfix increment returns a copy of its original, unincremented operand. If it returned the incremented value, we’d dereference the incremented value, with disastrous results. We’d skip the first element. Worse, if the sequence had no negative values, we would attempt to dereference one too many elements.