Header Include Order

The order of #include directives can matter in some cases, due to macro definitions, type definitions, or inline functions in headers.

The general rule is to include headers in the following order:

1. Related header: If a source file implements functions declared in a header file, include that header file first.
2. C system headers: Headers from the C standard library, such as <cstdio>.
3. C++ standard library headers: Such as <iostream> or <vector>.
4. Other libraries' headers: Headers from third-party libraries.
5. Your project's headers: Headers specific to your project.

Within each category, headers should be ordered alphabetically.

Here's an example:

1// my_class.cpp
2
3#include "my_class.h" // Related header
4
5#include <cstdio> // C system header
6
7#include <algorithm> // C++ standard library
8#include <iostream> // C++ standard library
9#include <vector> // C++ standard library
10
11#include <some_lib.h> // Other library
12
13#include "my_project.h" // Your project's header
14#include "utils.h" // Your project's header

This order ensures that the related header is included before anything else, system headers come before user headers, and within each category, headers are ordered alphabetically for easy scanning.

However, in most cases, the order of #include directives doesn't matter as long as the required declarations are available when needed. Modern compilers are pretty good at handling include order.