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The All-in-One Guide to C++20

Submodules and Module Partitions

Explore the use of submodules and module partitions in C++20 to divide large modules into manageable parts. Understand how submodules and partitions help structure code, enable fine-grained imports, and improve compilation efficiency while maintaining consistent client usage.

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When your module becomes bigger, you want to divide its functionality into manageable components. C++20 modules offer two approaches: submodules and partitions.

Submodules

A module can import modules and then re-export them.

In the following example, module math imports the submodules math.math1 and math.math2.

Javascript (babel-node)
// mathModule.ixx
export module math;
export import math.math1;
export import math.math2;

The expression export import math.math1 imports module math.math1 and re-exports it as part of the module math.

For completeness, here are the modules math.math1 and math.math2. I used a period to separate the module math from its submodules. This period is not necessary.

Javascript (babel-node)
// mathModule1.ixx
export module math.math1;          
export int add(int fir, int sec) { 
    return fir + sec;
}
Javascript (babel-node)
// mathModule2.ixx
export module math.math2;       
export {                     
    int mul(int fir, int sec) {
        return fir * sec;
    }
}

If you look carefully, you recognize a small difference in the export statements in the modules math. While ...