Combining C and C: Understanding #ifdef __cplusplus
When working on projects that integrate legacy C code with C , the interaction between the two languages can be confusing. This article explores how #ifdef __cplusplus is used to reconcile these differences.
The Basics of #ifdef __cplusplus
The #ifdef __cplusplus preprocessor directive is utilized to specify different code blocks for C and C compilers. When the C compiler encounters this directive, it recognizes that it is dealing with a C code block and applies C syntax rules. Conversely, when a C compiler encounters this directive, it skips the code block entirely.
Implementation
Typically, in C header files, #ifdef __cplusplus is placed at the top to wrap the code in an extern "C" block. This instructs the C compiler to preserve the C linkage for the enclosed functions and variables. The extern "C" block is then terminated at the bottom of the file.
Nested extern "C" Blocks
extern "C" blocks can be nested to handle different levels of C and C integration. For example, if a C header file includes another C header file, the second header file should also be wrapped in extern "C" provided __cplusplus is defined.
Limitations and Considerations
- Functions without prototypes in .cpp files will have C linkage unless they are enclosed in an extern "C" block.
- Wrapping code in nested extern "C" blocks with extra extern "C" directives does not have any effect.
- It is important to extern "C" headers for third-party C libraries to ensure proper linking.
Conclusion
Understanding how #ifdef __cplusplus works is crucial when integrating C and C code. By utilizing extern "C" blocks and correctly placing #ifdef __cplusplus directives, developers can ensure that the code compiles and links correctly, enabling smooth interoperability between the two languages.
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