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Module Interface Unit and Module Implementation Unit

Get details about how the module interface unit and module implementation unit work.

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When the module becomes bigger, you should structure it into a module interface unit and one or more module implementation units. Following the previously mentioned guidelines to structure a module, I will refactor the previous version of the math module.

Module interface unit

C++
// mathInterfaceUnit.ixx
module;                   
#include <vector>
export module math;       
export namespace math {
   int add(int fir, int sec);
   int getProduct(const std::vector<int>& vec);
}

  • The module interface unit contains the exporting module declaration: export module math (line 7).

  • The names add and getProduct are exported (lines 11 and 13).

  • A module can have only one module interface unit.

Module implementation unit

C++
// mathImplementationUnit.cpp
module math;
#include <numeric>
namespace math {
    int add(int fir, int sec) {
        return fir + sec;
    }
    int getProduct(const std::vector<int>& vec) {
        return std::accumulate(vec.begin(), vec.end(), 1, std::multiplies<int>());
    }
}

  • The module implementation unit contains non-exporting module declarations: module math; (line 3).

  • A module can have more than one module implementation unit.

Main program

C++
// client3.cpp
#include <iostream>
#include <vector>
import math;
int main() {
    std::cout << '\n';   
   
    std::cout << "math::add(2000, 20): " << math::add(2000, 20) << '\n';
    
    std::vector<int> myVec{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    
    std::cout << "math::getProduct(myVec): " << math::getProduct(myVec) << '\n';
    
    std::cout << '\n';
}

From the user’s perspective, the module math (line 6) is included and the namespace math was added.

When my explanations become compiler dependent, I put them in a separate tip box. This information is, in general, highly valuable if you want to try it out.

🔑 Building the executable with the Microsoft compiler

Manually building the executable includes a few steps.

cl.exe /c /experimental:module mathInterfaceUnit.ixx /EHsc      
cl.exe /c /experimental:module mathImplementationUnit.cpp /EHsc 
cl.exe /c /experimental:module client3.cpp /EHsc                 
cl.exe client3.obj mathInterfaceUnit.obj mathImplementationUnit.obj
  • Line 1 creates the object file mathInterfaceUnit.obj and the module interface file math.ifc.
  • Line 2 creates the object file mathImplementationUnit.obj.
  • Line 3 creates the object file client3.obj.
  • Line 4 creates the executable client3.exe.

For the Microsoft compiler, you have to specify the exception handling model (/EHsc), and enable modules: /experimental:module.

Finally, here is the output of the program:

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Execution of the program client3.exe