Is there a performance difference between i++ and ++i in C++?

Question

We have the question is there a performance difference between i++ and ++i in C?

What\'s the answer for C++?

Answer 1

The performance difference between ++i and i++ will be more apparent when you think of operators as value-returning functions and how they are implemented. To make it easier to understand what's happening, the following code examples will use int as if it were a struct.

++i increments the variable, then returns the result. This can be done in-place and with minimal CPU time, requiring only one line of code in many cases:

int& int::operator++() { 
     return *this += 1;
}

But the same cannot be said of i++.

Post-incrementing, i++, is often seen as returning the original value before incrementing. However, a function can only return a result when it is finished. As a result, it becomes necessary to create a copy of the variable containing the original value, increment the variable, then return the copy holding the original value:

int int::operator++(int& _Val) {
    int _Original = _Val;
    _Val += 1;
    return _Original;
}

When there is no functional difference between pre-increment and post-increment, the compiler can perform optimization such that there is no performance difference between the two. However, if a composite data type such as a struct or class is involved, the copy constructor will be called on post-increment, and it will not be possible to perform this optimization if a deep copy is needed. As such, pre-increment generally is faster and requires less memory than post-increment.