Standard way of importing modules

Question

I am currently trying to use c++ modules in a code that should compile both on Windows (MSVC) and Linux (Clang and/or GCC).

I am currently developping in Visual Studio and used the "Standard Conformance Mode" (/permissive-) to make my code as portable as possible.

However the following code:

import std.core;
int main()
{
    std::cout << "Hello, World! haha" << std::endl;

    std::vector<int> myVec{4};

    std::map<std::string, size_t> myMap;

    return 0;
}

Can not compile with the /permissive- flag. I get the following error:

E3223 Cound not find module file "std.core" for import

error C2664: 'int _CrtDbgReport(int,const char *,int,const char *,const char *,...)': cannot convert argument 4 from 'int' to 'const char *'

I tought "std.core" might be a windows-only thing so i tried the following (i saw it in many examples) :

import <iostream>;
import <vector>;
import <map>;

But it results in the following errors:

error C7612: could not find header unit for 'PATH_TO_VS\include\iostream'

error C7612: could not find header unit for 'PATH_TO_VS\include\vector'

error C7612: could not find header unit for 'PATH_TO_VS\include\map'

Note : There are actually files named "iostream", "vector", and "map" in PATH_TO_VS\include.

Therefore i'm wondering what is the standard way of importing c++ modules ? If "import std.core" is the standard way, why doesn't it compile with /permissive- ?

I am using Visual Studio 2019 (Community) and CMake.

Edit:

Sorry i forgot to tell my compiler flags:

/experimental:module
/std:c++latest
/W4
/WX
/permissive-
/MDd
/EHsc

The code compiles without /permissive-, but does not when it is set. I can't figure out why

Only the import<> syntax is part of C++20 for the standard library. (User-written modules can be used via import foo;.) — Davis Herring

Jeremiah van Oosten Jeremiah van Oosten · Accepted Answer · 2020-12-04T13:28:55

According to Microsoft Docs, import headers are not yet implemented. See https://docs.microsoft.com/en-us/cpp/cpp/modules-cpp?view=msvc-160#imported-header-files.

You can following the progress on this feature here: https://github.com/microsoft/STL/issues/60.

You can use the import std.core; syntax in Visual Studio 2019 (I tested this with v16.8+) but you will also need to install the "C++ Modules for v142 build tools" component in the Visual Studio Installer for this to work.

In addition, you will need to enable the following flags:

/std:c++latest
/experimental:module

As stated in this answer.

You may still get some C5050 warnings about incompatible environment while importing the std.core module:

1>C:\Test\C++\Modules\main.cpp(1,16): warning C5050: Possible incompatible environment while importing module 'std.core': _GUARDOVERFLOW_CRT_ALLOCATORS=1 is defined in current command line and not in module command line
1>C:\Test\C++\Modules\main.cpp(1,16): warning C5050: Possible incompatible environment while importing module 'std.core': _DEBUG is defined in current command line and not in module command line
1>C:\Test\C++\Modules\main.cpp(1,16): warning C5050: Possible incompatible environment while importing module 'std.core': _M_FP_PRECISE is defined in current command line and not in module command line

To solve the first warning change SDL Checks to No (/sdl-).
To solve the second warning, remove the _DEBUG preprocessor definition.
To solve the third warning, delete the value of the Floating Point Model (which was, by default, set to /fp:percise in my case).

Standard way of importing modules

2 Answers