Minimal demo of the pimpl idiom using C++20 modules

CMakeLists.txt

cmake_minimum_required(VERSION 3.28)
project(module-pimpl CXX)

set(CMAKE_CXX_EXTENSIONS Off)

add_library(pimpl)
target_compile_features(pimpl PUBLIC cxx_std_20)
target_sources(pimpl PUBLIC
    FILE_SET CXX_MODULES
    BASE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}
    FILES pimpl.cpp
)
target_sources(pimpl PRIVATE pimpl-impl.cpp)

add_executable(test test.cpp)
target_link_libraries(test PRIVATE pimpl)

pimpl-impl.cpp

module;

#include <string>
#include <memory>

module pimpl;

namespace pimpl {

struct TestPrivate {
    std::string greeting = "Hello world";
};

Test::Test()
    : priv_(std::make_unique<TestPrivate>())
{}

Test::~Test() = default;

const std::string& Test::greeting() const
{
    return priv_->greeting;
}

}

pimpl.cpp

module;

#include <memory>
#include <string>

export module pimpl;

namespace pimpl {

class TestPrivate;

export class Test {
public:
    explicit Test();
    ~Test();

    const std::string& greeting() const;

private:
    std::unique_ptr<TestPrivate> priv_;
};

}

test.cpp

#include <iostream>

import pimpl;

int main()
{
    pimpl::Test t;

    std::cout << t.greeting() << std::endl;
}