In C++20, the introduction of modules has brought a new way of organizing and packaging code. Modules provide a more efficient and scalable alternative to traditional header files. However, it is important to ensure that existing codebases with header files can continue to work seamlessly with modules. In this article, we will explore techniques for supporting interoperability between C++ modules and traditional header files.

Importing Header Files in Modules

One of the challenges when migrating to modules is dealing with existing header files. Fortunately, C++20 allows us to import header files directly into modules using the import keyword. This enables modules to access the declarations and definitions in the header files.

To import a header file, we can use the following syntax:

import "path/to/header_file.h";

By importing the header file, its declarations and definitions become accessible within the enclosing module. This allows us to gradually transition from header files to modules without rewriting all the code at once.

Exporting Declarations from Modules

When migrating from header files to modules, it is necessary to export the declarations that were previously available in the header files. This is important for other modules or client code that rely on these declarations.

To export a declaration from a module, we can use the export keyword followed by the declaration:

export void myFunction();

The export keyword ensures that the declaration is visible to other modules and client code. This helps maintain backward compatibility and allows modules to provide the necessary interface to the outside world.

Mixing Modules and Header Files

In some cases, it may be desirable to mix modules and header files within the same codebase. For example, when working with third-party libraries that have not yet migrated to modules. C++20 allows us to mix modules and header files to achieve interoperability.

To use code from a header file within a module, we need to import the header file as described earlier. This allows the module to access the declarations and definitions in the header file.

Conversely, to use code from a module within a codebase using traditional header files, we need to provide a bridging header file. The bridging header file includes the necessary import statements for the modules, making their symbols available to the codebase using header files.

Conclusion

Supporting interoperability between C++ modules and traditional header files is crucial for migrating existing codebases and working with legacy code or third-party libraries. By using the appropriate import and export keywords, we can integrate modules and headers seamlessly, allowing for a gradual transition to the new module-based approach while maintaining backward compatibility.

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