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Modules

C++20 introduced modules as a significant enhancement to the language, aiming to replace the traditional header-file mechanism. Here's a comprehensive overview of what modules are and how they function in C++20:

  1. Purpose and Advantages:

    • Modules are intended as an alternative to header files, designed to overcome the limitations of the #include preprocessor directive 1.
    • By replacing headers, modules aim to reduce compilation times, isolate macros, and eliminate the need for include guards or pragma once1.

  2. How They Work:

    • They turn C++ libraries and programs into components, where each module is a set of source code files compiled separately from the source files that import them 2.
    • This independence helps to eliminate or significantly reduce the problems traditionally associated with header files, such as repetitive inclusion and compilation overhead.

  3. Structure of Modules:

    • A module unit is a source file containing a module declaration. There are two primary types of module units:
      • Module Interface Unit: It exports a module name with the export keyword and specifies which symbols (functions, classes, variables) are available to other parts of the program.
      • Module Implementation Unit: It is used to implement the functionality of the modules and does not export symbols directly 2.

  4. Usage:

    • In practice, a module is similar to a header but also acts as a translation unit. This means that it is compiled separately and only once, which can improve build times and reduce redundancy.
    • You can use the import keyword, a new addition to C++20, to gain access to declarations from a module, much like you would include a header file to access its declarations.

  5. Sharing Across Translation Units:

    • Since many C++ projects consist of multiple translation units, modules provide a way to share declarations and definitions across these units without the pitfalls of header file.

In summary, C++20 modules represent a paradigm shift in how code is organized and shared across different parts of a program, promising to make C++ development more efficient by enhancing compilation times and modularizing codebases.