Combine std module with concurrent/ppl

Question

I am trying to convert a header file into a module by compiling the following:

// def.ixx
export module def;

#include <concurrent_unordered_map.h>
import std;

export using dictionary = std::unordered_map<std::string, std::variant<double, std::string, bool, std::vector<double>, int, std::size_t>>;
export using results = std::unordered_map<std::string, std::vector<double>>;
export using pool_results = std::unordered_map<std::string, std::vector<std::vector<std::vector<std::vector<double>>>>>;
export using cache = concurrency::concurrent_unordered_map<std::size_t, results>;
export using pool_cache = std::unordered_map<std::size_t, pool_results>;

I have previously compiled the std module. I am compiling with cl /std:c++latest /EHsc /nologo /reference "std=std.ifc" def.ixx and get the following error:

def.ixx
def.ixx(5): warning C5244: '#include <concurrent_unordered_map.h>' in the purview of module 'def' appears erroneous.  Consider moving that directive before the module declaration, or replace the textual inclusion with 'import <concurrent_unordered_map.h>;'.
def.ixx(3): note: see module 'def' declaration
LINK : fatal error LNK1561: entry point must be defined

I have tried as it suggests, however, a new error appear. Is it possible to use std as a module and also use concurrency/concurrent_unordered_map?

Answer 1

The correct way to use include with module is to put them in the global module fragment:

// def.ixx
module; // global module fragment
#include <concurrent_unordered_map.h> // don't claim ownership of the declarations

export module def;
import std;

export using dictionary = std::unordered_map<std::string, std::variant<double, std::string, bool, std::vector<double>, int, std::size_t>>;
export using results = std::unordered_map<std::string, std::vector<double>>;
export using pool_results = std::unordered_map<std::string, std::vector<std::vector<std::vector<std::vector<double>>>>>;
export using cache = concurrency::concurrent_unordered_map<std::size_t, results>;
export using pool_cache = std::unordered_map<std::size_t, pool_results>;

Every translation units that needs to include this header will have to re-parse/re-compile the same header, as this is textual inclusion, as opposed to import header.

---

To instead use header units, you must precompile it first:

cl /nologo /EHsc /std:c++latest /exportHeader path/to/concurrent_unordered_map.h

Then reference it when compiling:

cl /std:c++latest /EHsc /nologo /reference "std=std.ifc" def.ixx /headerUnit concurrent_unordered_map.h=concurrent_unordered_map.h.ifc