'Can't use iostream as module in C++20 (Visual Studio)

Can't get this code running in 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 similar question here Standard way of importing modules):

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

Can anybody help me with this? Should i use Clang or GCC instead?

c++c++20


Solution 1:[1]

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 book's page for details and corresponding github repo).

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

HTH.

Regards, Glenarvan

Solution 2:[2]

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

Solution 3:[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 )

Solution 4:[4]

Try to do like that: import <iostream>; // import declaration

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

Example:

helloworld.cpp

export module helloworld;  // module declaration
import <iostream>;         // import declaration
 
export void hello() {      // export declaration
    std::cout << "Hello world!\n";
}

main.cpp

import helloworld;  // import declaration
 
int main() {
    hello();
}

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Glenarvan
Solution 2 Gass
Solution 3 Charles Chan
Solution 4 bogdyname

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