Should C Definitions Always Be Placed in Header Files?

C Header Files: Definition Placement

Your coworker's assertion that all C declarations and definitions should be placed in header files is unfounded. The common practice in C , historically and currently, remains to separate code definitions into .cpp files and declarations in header files.

Reasons for Definition Separation

• Control Over Compile Times: Compiling all code in headers significantly increases compile times, especially when numerous files include the same headers. This becomes particularly problematic for large projects.
• Avoidance of Circular Object Relationships: When code is placed in headers, it can lead to circular object dependencies, which can be difficult to resolve.
• Limited Scope for Global Objects: Header-only code cannot contain simple global objects due to the risk of multiple definitions when included in multiple source files.

Exception: Templates

The notable exception to this rule is templates. Modern C libraries like Boost make extensive use of templates, which require definition in headers to allow for inlining optimizations. However, this practice should be limited to templates only.

Downsides of Header-Only Code

While header-only code can be convenient for templates, it has several drawbacks:

• Increased compile times, making development and refactoring slower.
• Limited functionality, as some features (e.g., global objects) cannot be implemented in header-only code.
• Potential for recompilation issues, as changes require recompiling the entire project.

Conclusion

Despite your coworker's claims, the separation of definitions into .cpp files and declarations in header files remains the preferred approach in C for maintaining code organization, optimizing compile times, and avoiding potential pitfalls.

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