不能在 C++20 (Visual Studio) 中使用 iostream 作为模块

Question

I can't get this code running in the newest version of MSVC . This code example is from the book called "Beginning C++20, From Novice to Professional" by Ivor Horton and Peter Van Weert.

import <iostream>;

int main()
{
    int answer {42};
    std::cout << "The answer to life, universe, and everything is "
              << answer
              << std::endl;
    return 0;
}

I get this error:

could not find header unit for 'C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.28.29333\include\iostream'

I am using Microsoft Visual Studio version 16.8.1, and have enabled these flags in project properties (according to the similar question Standard way of importing modules ):

• /experimental:module
• /std:c++latest
• /EHsc
• /MD

Should I use Clang or GCC instead?

Answer 1

Using the Visual Studio 2019 non preview version:

1. Create an empty C++ project

2. Open project properties: Alt + Enter

3. Go to Configuration Properties → C/C++ → Language , and set the C++ Language Standard option to Preview - Features from the Latest C++

4. In the same section, set Enable Experimental C++ Standard Library Modules to Yes (/experimental:module)

5. Go to Configuration Properties → C/C++ → Advanced and set the Compile As option to Compile as C++ Module Internal Partition (/internalPartition)

6. Add header file to your project, which contains an import declaration for every standard library header you want to import. For example:

    #pragma once import <iostream>; import <array>; import <vector>;

7. Recompile your project

8. Done, now everything should work fine

Answer 2

The authors stated in the example and solution files provided for download, that due to compilers' implementations of the C++20-standard the files might not work yet (see the book's page for details and corresponding GitHub repository ).

Therefore they provide "no modules" examples which use the "#include" directive.

Answer 3

Modules in C++20 (Visual Studio)

If you are using “modules”:

This will not work: 5. Go to Configuration Properties → C/C++ → Advanced and set the Compile As option to Compile as C++ Module Internal Partition (/internalPartition)

This will work: 5. Go to Configuration Properties → C/C++ → Advanced and set the Compile As option to Compile as C++ Module Code (/interface)

Answer 4

Try to do it like this:

import <iostream>;         // import declaration

Modules help divide large amounts of code into logical parts. Modules are orthogonal to namespaces.

Example :

File helloworld.cpp

export module helloworld;  // module declaration
import <iostream>;         // import declaration

export void hello() {      // export declaration
    std::cout << "Hello, World!\n";
}

File main.cpp

import helloworld;  // import declaration

int main() {
    hello();
}