Gettings C7621 when trying to use C++ 20 modules

Question

I'm following this example of how to write modules in c++ 20: https://learn.microsoft.com/en-us/cpp/cpp/tutorial-named-modules-cpp?view=msvc-170 and I continue getting an error C7621 "module partition 'XXX' for module unit 'XXX' was not found.

Following the example I've created the next ixx *primary module interface:

export module Chapter01;

export import :Question01;

I've then added the ixx module partition file:

export module Chapter01:Question01;

import <iostream>;
import <set>;

using namespace std;

export bool IsUnique(const std::string& str);

And the cpp implementation file:

module Chapter01:Question01;

import <iostream>;
import <set>;

using namespace std;

bool IsUnique(const std::string& str)
{
    set<char> charset = set<char>();

    for (const char c : str)
    {
        if (charset. Contains(c))
        {
            return false;
        }

        charset. Insert(c);
    }

    return true;
}

And I keep getting a C7621: module partition 'Question01' for module unit 'Chapter01' was not found

Looking at the example over and over again I can't seem to find the difference.

Removing the cpp file entirely and putting the implementation in the ixx file fixes the issue, but I would still like to understand why the separation doesn't work.

P.S. What is the best practice when working with modules? To separate the ixx and cpp files or to just include ixx files?

Answer 1

The tutorial you're using is incorrect. It suggests that you can have an implementation partition and interface partition that use the same partition name in the same module.

This is not allowed. All partitions within the same module must have different partition names.

Furthermore, the only reason to have an implementation partition is if you want some other module unit to be able to import that partition. Most implementation partitions don't need to be imported. So your implementation unit should just use module Chapter01;, and it will automatically import the Chapter01 module.