I’m trying to use modules to speed up our windows builds a little bit.
From my first impressions C++20 Modules are definitely faster than PCH (atleast on windows).

Currently I’m only focussed on windows with latest MSVC 17 and latest CMake. I’m also using import std which is only available on C++23.
I know that these features are cutting edge, but this is a first try and not used in any productive system. (I need to preserve backwards compatibility though with “classic” includes)
Since MSVC17 has a good working module support and CMake has added the import std in their latest version I’d like to give it a try.

Now the problems I have encountered are name clashes.
So basically since I use import std; I would like to avoid all stl includes.
Modifying our code is not a big deal, the real challenge is packaging existing 3rd Parties (like fmt and nlohmann_json) into modules since their headers include multiple std headers (which are causing name clashes with import std;)

I have watched Daniela Engert talks about C++20 modules from some CppCons and checked out her examples.
https://github.com/DanielaE/asio/tree/d212efd69aedf221475dc2434c45a6baeee10d16/asio/module
She basically included the 3rd party header within the export. However this did not work for me.

// Global module fragment where #includes can happen
module;

//inlcuded headers will not be exported to the client


// first thing after the Global module fragment must be a module command
export module nlohmann_json;

extern "C++"
{
  export
  {
#include <nlohmann/json.hpp>
  }
}


module: private;
//possbile to include headers here, but they will not be exported to the client

This is my test programm

#include <gtest/gtest.h>
TEST(CxxModuleTest, nlohman_json)
{
  nlohmann::json j = {
  {"pi", 3.141},
  {"happy", true},
  {"name", "Niels"},
  {"nothing", nullptr},
  {"answer", {
    {"everything", 42}
  }},
  {"list", {1, 0, 2}},
  {"object", {
    {"currency", "USD"},
    {"value", 42.99}
  }}
  };

  EXPECT_EQ(j["pi"], 3.141);
  EXPECT_EQ(j["happy"], true);
  EXPECT_EQ(j["name"], "Niels");
  EXPECT_EQ(j["nothing"], nullptr);
  EXPECT_EQ(j["answer"]["everything"], 42);
  EXPECT_EQ(j["list"][0], 1);
  EXPECT_EQ(j["list"][1], 0);
  EXPECT_EQ(j["list"][2], 2);
  EXPECT_EQ(j["object"]["currency"], "USD");
  EXPECT_EQ(j["object"]["value"], 42.99);
}

However this does not compile. I’m getting

'#include <filename>' in the purview of module 'module-name-1' appears erroneous. Consider moving that directive before the module declaration, or replace the textual inclusion with 'import <filename>;'.

I ended up with this

// Global module fragment where #includes can happen
module;

//inlcuded headers will not be exported to the client
#include <nlohmann/json.hpp>

// first thing after the Global module fragment must be a module command
export module nlohmann_json;

export
{
  namespace nlohmann::json_abi_v3_11_2::detail
  {
    using ::nlohmann::json_abi_v3_11_2::detail::json_sax_dom_callback_parser;
    using ::nlohmann::json_abi_v3_11_2::detail::json_sax_dom_parser;
  }

  namespace nlohmann
  {

    using ::nlohmann::basic_json;
    using ::nlohmann::json;
    using ::nlohmann::json_sax;
    using ::nlohmann::detail::json_sax_dom_parser;
    using ::nlohmann::detail::json_sax_dom_callback_parser;
  }
}


module: private;
//possbile to include headers here, but they will not be exported to the client

However this is a try and error. I need to add all types until the program compiles.

=>My question:
Is there a better (more generic) way?

thx guys for your help 🙂