How Can I Effectively Use Precompiled Headers with GCC, and What Are Its Limitations?

GCC and Precompiled Headers

When working with precompiled headers (PCHs) in GCC, it's crucial to understand their limitations. PCHs in GCC require a single common header included as the first non-comment lexeme in every source file. This header includes all necessary system and library-specific headers.

The following scenarios can disrupt the PCH's effectiveness:

1. Redefining Macros in .cpp Files: Defining macros that alter the preprocessor's interpretation of headers in the PCH can break its functionality.
2. Including Headers with Directive Redefinitions: Including headers that #define specific preprocessor directives can also invalidate the PCH, especially if such directives affect headers already included in the PCH.
3. Recursive Header Inclusions: Including headers that #include other headers can lead to complex interactions and potentially cause PCHs to fail.

GCC's limitations mean that strict coding practices are required when using PCHs, such as:

• Limiting header inclusions in .cpp files to one.
• Avoiding #define statements in .cpp files.

Detect rebuild triggers for PCHs in project builds can be challenging. While usage of -H to obtain GCC's inclusion information can be helpful, it doesn't guarantee accurate PCH rebuild detection.

GCC's current model requires a single precompiled header. Efforts to improve this approach, namely the PreParsed Header (pph) branch, have been abandoned. Future C versions may provide a module mechanism to address these limitations.

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