When Do I Need to Rebuild My GCC Precompiled Headers?

GCC and Precompiled Headers: When to Rebuild

In the article "The Care and Feeding of Pre-Compiled Headers," it is not entirely clear when it becomes necessary to rebuild a precompiled header. This article delves into specific scenarios where rebuilds may be required, addressing potential pitfalls and suggesting guidelines for avoiding them.

GCC Limitations

GCC currently requires a single common header for an application (#include-ed in every source file with identical preprocessing options). This single header typically includes necessary system and library headers.

Scenarios Requiring Rebuilds

Rebuilds of the precompiled header are necessary in certain situations:

• Changes to Macros: Defining macros in .cpp files can alter how the preprocessor interprets pre-included headers, requiring a rebuild.
• Additional Headers: Including headers that define macros affecting pre-included headers necessitates a rebuild.
• Recursive Inclusions: Headers that recursively include others can cause a domino effect, requiring multiple rebuilds.

Constraining Coding Style

To minimize rebuilds, GCC's precompiled header functionality may impose a restrictive coding style:

• Limit headers in .cpp files to one.
• Avoid defining macros in .cpp files.

Manual Handling

For complex projects with multiple libraries, detecting header modifications for rebuild purposes involves intricate Makefile manipulations.

Future Enhancements

GCC's limitations may improve with ongoing efforts on the PreParsed Header branch. Additionally, future C standards may define module mechanisms that address these issues.

Conclusion

By adhering to GCC's specific requirements, developers can effectively utilize precompiled headers. However, limitations remain, and future enhancements or standardization may provide more robust solutions for managing precompiled header dependencies.

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