Modules are better than header files because they solve problems such as duplicate inclusion, slow compilation speed, and macro pollution. 1. Modules adopts a mechanism of compiling once and importing multiple times, avoiding the problem of re-parsing header files every time they are compiled, improving construction efficiency; 2. The modules are naturally isolated from interfaces and implementations, reducing naming conflicts and unnecessary symbol exposure. Writing a simple module requires an interface unit and an implementation unit: 1. The interface unit starts with export module name; and exports the declaration; 2. The implementation unit starts with module name; and contains specific implementation code. Note when using modules: 1. The module interface must be compiled before the code that references it; 2. The existing header file code base cannot be directly converted, and the migration cost is high; 3. There are differences in support from different compilers, and some characteristics may be unstable. In the short term, header files will still be mainstream, but as the tool chain is improved, modules will gradually replace header files and become the mainstream method.

C 20 introduces modules, a major improvement to the traditional header file mechanism. It solves multiple problems caused by header files, such as duplicate inclusion, slow compilation speed and macro pollution. Although header files have been used for a long time in the development of C, the emergence of modules has made the code organization clearer and faster to build.

Why are modules better than header files?

Traditional header files copy content through #include preprocessing directives, which is simple but inefficient. The header file contents must be re-parsed every time the compilation is completed, resulting in the compilation time of large projects becoming longer. modules are a mechanism for compiling once and importing multiple times, avoiding repeated parsing.

In addition, header files are prone to naming conflicts, especially macro definitions. The module naturally isolates the interface and implementation, and does not expose unnecessary symbols.

How to write a simple module?

The basic structure of a writing module includes module interface unit and implementation unit:

• Interface unit : Start with export module name; and declares exportable content.
• Implementation unit : Start with module name; including specific implementations.

For example:

 // math.ixx (interface unit)
export module math;

export int add(int a, int b);

// math.cpp (implementation unit)
module math;

int add(int a, int b) {
    return ab;
}

Then other files can be used like this:

 import math;

int main() {
    return add(1, 2);
}

Note that module files usually use .ixx or .cppm extensions (depending on the compiler), and require compilers that support C 20 such as MSVC or GCC 11.

What should you pay attention to when using modules?

• There are some requirements for the compilation order : the module interface must be compiled before the code that references it. Otherwise, the module will not be found.
• Incompatible with old code : There are already a large number of header file-based code libraries that cannot be directly converted into modules, and the migration cost is high.
• There are slight differences in support for different compilers : GCC and Clang support for modules is still being improved, and some features may be unstable.

It is recommended to start with new projects or small functional modules and gradually transition to modular development.

How long will the header file last?

In the short term, header files will remain mainstream. Many existing projects rely on header files, and the standard library is not fully modular yet. However, with the improvement of the tool chain, modules will gradually replace header files in the future and become the recommended way.

Basically that's it. The module is not particularly complicated, but it is necessary to pay attention to the compilation process and compatibility issues in actual use.

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