Why Do We Put Function Implementations in Header Files?

Header Files: Functions vs. Implementations

Many programmers assume that header files exclusively declare functions, with their implementations stored in separate C/CPP files. However, this is not always the case, as exemplified by the following code snippet:

public:
    UInt32 GetNumberChannels() const { return _numberChannels; } // <-- Huh??

This code defines a class method (GetNumberChannels) within its corresponding header file. But why? Let's dive deeper into the purpose of header files and explore the benefits of this practice.

Purpose of Header Files

Header files facilitate code sharing among multiple source files. However, they can also house function implementations. When the preprocessor encounters an #include statement, it replaces it with the contents of the referenced file, resulting in a single preprocessed code that the compiler then processes.

Implementation within Headers

By including method implementations in header files, they are implicitly marked as inline. This is not a guarantee of function inlining, but if inlined, the function's contents are copied directly into the call site where it is used, improving code optimization.

Benefits

Inlining functions can result in two primary benefits:

1. Faster Execution: By eliminating CALL and return statements, the compiler can optimize the copied code more effectively.
2. Improved Optimization: The compiler has access to the surrounding code, allowing it to further optimize the inlined code and improve overall performance.

Alternatives

While inlining functions within header files is a common practice, it is not always necessary. Alternatively, you can define function implementations in separate C/CPP files to improve code organization and enhance readability.

Conclusion

Understand that header files serve as a hub for code sharing, and implementations within headers can be implicitly declared as inline. This technique may yield performance advantages, but its effectiveness is compiler-dependent. As always, consider the specifics of your project and optimize accordingly.

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