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

c++, oop, performance, post-increment, pre-increment

Solution

[Executive Summary: Use `++i` if you don't have a specific reason to use `i++`.]

For C++, the answer is a bit more complicated.

If `i` is a simple type (not an instance of a C++ class), then the answer given for C ("No there is no performance difference") holds, since the compiler is generating the code.

However, if `i` is an instance of a C++ class, then `i++` and `++i` are making calls to one of the `operator++` functions. Here's a standard pair of these functions:

Foo& Foo::operator++()   // called for ++i
{
    this->data += 1;
    return *this;
}

Foo Foo::operator++(int ignored_dummy_value)   // called for i++
{
    Foo tmp(*this);   // variable "tmp" cannot be optimized away by the compiler
    ++(*this);
    return tmp;
}

Since the compiler isn't generating code, but just calling an `operator++` function, there is no way to optimize away the `tmp` variable and its associated copy constructor. If the copy constructor is expensive, then this can have a significant performance impact.

Problem

Original source

Related problems

• Performance difference between ++iterator and iterator++?
• Is there a performance difference between i++ and ++i in C?
• Int Argument in operator++