前言

Modules 的好處

Compiler 支持狀況

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[/quote]

配置 Visual Studio

在開始之前

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[/quote]

Prerequisites

Project Properties

• C++ Language Standard 至少要設定 [code]/std:c++20[/code]
  Configuration Properties > General > C++ Language Standard
• Scan Sources for Module Dependencies 設定成 [code]Yes[/code]
  Configuration Properties > C/C++ > General

概述

[code]// hello.ixx
export module helloworld; // module declaration

import <iostream>;        // import declaration

export void hello()       // export declaration
{
    std::cout << "Hello world!\n";
}
[/code][code]// main.cpp
import helloworld; // import declaration

int main()
{
    hello();
}
[/code][code]// Console
Hello world!
[/code]

hello.ixx

• [code]export module helloworld;[/code]
  宣告並匯出一個 module , 名字為 helloworld ，
  加上 [code]export[/code] 代表這是一個 primary module interface unit
• [code]import <iostream>;[/code]
  以 [code]header unit[/code] 的方式 [code]import[/code] 一個 header
• [code]export void hello();[/code]
  要匯出一個 function , 在宣告前加上 [code]export[/code] 即可

main.cpp

• [code]import helloworld;[/code]
  [code]import[/code] 一個 module , 名字為 helloworld， module 的名字為前面 [code]export module[/code] 的名稱，與檔名無關

Module declarations - 宣告 Module

[code]// hello.ixx
export module helloworld; // module declaration

import <iostream>;        // import declaration

export void hello();     // export declaration
[/code][code]// hello.cpp
module helloworld; // declares a module implementation unit for named module 'helloworld'

// export void hello() // ERROR: a declaration can be exported only from a module interface unit
void hello()       // implementation for 'void hello()'
{
    std::cout << "Hello world!\n";
}
[/code][code]// main.cpp
import helloworld; // import declaration

int main()
{
    hello();
}
[/code]

• module interface unit
• module implementation unit
• primary module interface unit
• module partition interface unit
• module partition implementation unit

module interface unit

module implementation unit

[quote]

[/quote]

module partition units

primary module interface unit

dot 句點

Exporting declarations - 匯出宣告

[code]// hello.ixx
export module helloworld; // declares the primary module interface unit for named module 'helloworld'


// zero() will be visible by translations units importing 'helloworld'
export int zero() { return 0; }

// one() will NOT be visible.
int one()  { return 1; }

// Both two() and three() will be visible.
export
{
    int two()  { return 2; }
    int three() { return 3; }
}

// Exporting namespaces also works: number::four() and number::five() will be visible.
export namespace number
{
    int four()  { return 4; }
    int five() { return 5; }
}
[/code]

Importing modules and headers - 匯入 modules 跟 headers

modules and headers

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export-import

[code]// A.ixx (primary module interface unit of 'A')
export module A;

import <iostream>; // import headeer
export import <string_view>; // export-imports

export void print(std::string_view message)
{
    std::cout << message << std::endl;
}
[/code][code]// main.cpp
import A; // import module

int main()
{
    std::string_view message = "Hello, world!";
    print(message);
}
[/code]

Global module fragment

[code]// A.ixx (primary module interface unit of 'A')
module; // start

// https://learn.microsoft.com/en-us/cpp/c-runtime-library/math-constants?view=msvc-170
// Defining _USE_MATH_DEFINES to use Math Constants
#define _USE_MATH_DEFINES // for C++
#include <cmath>

export module A; // end

import <iostream>;

export void pi()
{
    std::cout << M_PI << std::endl; // 3.1415926
}
[/code]

Private module fragment

[code]export module A;

export int one();

module : private; // The start of the private module fragment.

int one()           
{
    return 1;
}
[/code]

Module partitions - 模組分區

• 命名習慣上，通常是 [code]<primary-module-name>-<module-partition-name>[/code] , ( e.g, [code]A-B[/code] , [code]A-C[/code] )
• 一個 module partition 只能屬於一個 module，([code]export module A:B;[/code] 屬於 [code]A[/code] )
• module partition 可以被其他 partition [code]import[/code] , 語法為 [code]import :part-name[/code]

[quote]

[/quote]
[code]// A.ixx
export module A;     // primary module interface unit

export import :B;    // Hello() is visible when importing 'A'.
export import :C;    // WorldImpl() is visible only for 'A.ixx'.

// World() is visible by any translation unit importing 'A'.
export void World()
{
    std::cout << WorldImpl() << '\n';
}

export int zero()
{
    return 0;
}
[/code][code]// A-B.ixx
export module A:B; // partition module interface unit

// import :C;
import <iostream>;

// Hello() is visible by any translation unit importing 'A'.
export void Hello()
{
    std::cout << "Hello" << '\n';
    // std::cout << WorldImpl() << '\n'; // ERROR: WorldImpl() is not visible.
}
[/code][code]// A-C.ixx
export module A:C; // partition module interface unit

// WorldImpl() is visible by any module unit of 'A' importing ':C'.
char const* WorldImpl() { return "World"; }
[/code][code]// main.cpp
import A;

// import <iostream>;

int main()
{

    Hello();
    World();

    // std::cout << zero() << '\n'; // ERROR: 'cout': undeclared identifier
    // WorldImpl(); // ERROR: WorldImpl() is not visible.
}
[/code]
[quote]

[/quote]
[code]//  A.cpp   
export module A;     // primary module interface unit

import :C;           // WorldImpl() is now visible only for 'A.cpp'.
// export import :C; // ERROR: Cannot export a module implementation unit.
[/code][code]// A-C.cpp
module A:C; // partition module **implementation** unit

// WorldImpl() is visible by any module unit of 'A' importing ':C'.
char const* WorldImpl() { return "World"; }
[/code]

結語

References

• Overview of modules in C++ | Microsoft Learn
• Modules (since C++20) - cppreference.com
• C++20: Structure Modules - ModernesCpp.com
• visual c++ - C++ 20 experimental std Modules not accessible with latest MSVC experimental tools enabled - Stack Overflow
• Walkthrough: Build and import header units in Visual C++ projects | Microsoft Learn
• [C++20 Modules] Module Partition Implementation Unit Bug(?) - Visual Studio Feedback
• Understanding C++ Modules: Part 1: Hello Modules, and Module Units (vector-of-bool.github.io)
• [module.unit] (eel.is)
• No Diagnostic Required - cppreference.com

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