C Modules is a feature introduced by C 20 to improve compilation efficiency and code organization. 1. It replaces #include with import, compiling the module into an intermediate format (such as .pcm file) to avoid repeated parsing of source code; 2. The module is divided into interface unit (export module) and implementation unit, supporting clearer interface control and encapsulation; 3. Using modules can reduce dependency transfer, improve compilation speed, and enhance code security; 4. At present, you need to pay attention to problems such as inconsistency in the compiler support, construction process adjustment, and .pcm file management.

C modules is a major language feature introduced by C 20, aiming to replace the traditional header file mechanism and improve compilation efficiency, code organization and maintainability. If you have written a large C project, you may encounter problems such as slow compilation and complex header file dependencies, and modules are designed to solve these problems.

Let’s take a look at several key points of C modules and how to use it to improve your project structure.

What are C Modules?

Simply put, C modules is a modular way of code that allows developers to separate interfaces and implementations and compile them in a more efficient way.

In the traditional way, we import header files through #include , which will lead to duplicate parsing, macro pollution, dependency chain length and other problems. modules use import instead of #include . The content of the module will be processed into an intermediate format (usually a .pcm file) when compiled. In subsequent use, only the intermediate result needs to be loaded without re-parsing the source code.

for example:

 // math.cppm
export module math;

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

Then use it in another file:

 // main.cpp
import math;

#include <iostream>

int main() {
    std::cout << add(3, 4) << std::endl;
    return 0;
}

How to build a module?

To use C modules, you must first understand the basic structure and construction process of the module.

Modules are usually divided into interface units and implementation units. The interface unit uses the export module declaration to define the interface exposed by the module; the implementation unit contains specific implementation logic.

The steps for building a module are roughly as follows:

• Write module interface ( .cppm or .ixx file)
• Write module implementations (can be .cpp or .cppm )
• Compile the module interface to generate .pcm file
• Compile module implementation and link
• Use import in the main program to import the module and compile it

Different compilers (such as MSVC, GCC, Clang) have slightly different support for modules, and the compilation commands are also different. For example, building a module with MSVC might require:

 cl /std:c latest /EHsc /c math.cppm /FoMath.obj
cl /std:c latest /EHsc main.cpp Math.obj /Femain.exe

What benefits can Modules bring?

The biggest advantage is the improvement in compilation speed . Because the module only needs to be compiled once, the compiled interface is only required to load the subsequent import, rather than repeating the parsing like the header file.

There are several other obvious advantages:

• Clearer interface control : Only export -marked content will be exposed to the outside world to avoid naming pollution.
• Reduce dependency pass : Dependencies between modules are clearer, and header file nesting explosions are not prone to occur.
• Better encapsulation : The internal implementation of the module can be hidden to improve code security.

For example, if you write a module A, only one function foo() is exported, then even if you use the function bar() in the module, you won't see it outside unless you actively export .

Notes on using Modules

Although modules are powerful, there are still some restrictions and things to note:

• Not all compilers fully support C 20 modules, especially the implementation of GCC and Clang is still being improved.
• Compatibility issues may occur when the module is mixed with traditional header files.
• The build process needs to be redesigned, especially the project structure and build system (such as CMake) require adaptation of module mechanisms.
• The management of .pcm files requires additional attention, and the formats of different compilers are incompatible.

It is recommended to try using modules in new projects, or gradually migrate modules with strong independence in old projects.

Basically that's it. C modules don't seem complicated, but there are some details that are easy to ignore in actual use, especially in cross-platform and build processes. If you are just starting to get in touch, you can practice from a small module and gradually master the structure and compilation methods of the module.

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