CODEWITHSUNDEEP
c++modulenamespacesc++20constexpr
v.p.
v.p.

Reputation: 98

C++20 Module with Namespace

I am hoping to receive some input on an issue I encountered while attempting to learn c++20 modules.
In short, I would like a namespace containing const and/or constexpr variables to be implemented within a module, and import that module into any implementation files applicable. It works fine with non const / non constexpr variables, however this isn't ideal; I'd like to stick with const and/or constexpr depending on the data types within the namespace.

Please see example below:

// test_module.cpp
module;
#include <string_view>

export module test_module;

export namespace some_useful_ns {
        /* produces a compile error when attempting to compile main.cpp */
        constexpr std::string_view str_view{"a constexpr string view"};
        const char* const chr{"a const char* const"};
        
        /* compiles just fine, but not ideal; prefer const / constexpr
        std::string_view str_view{"a non constexpr str view"};
        const char* chr{"a const char*"};
        */
}

// main.cpp
#include <iostream>

import test_module;

int main() {
        std::cout << some_useful_ns::str_view << std::endl;
        std::cout << some_useful_ns::chr << std::endl;
        return 0;
}

Compile the two files like below:

g++ -c --std=c++2a -fmodules-ts test_module.cpp
g++ -c --std=c++2a -fmodules-ts main.cpp
g++ main.o test_module.o

Before linking the .o files, I receive the following error when compiling main.cpp like above:

main.cpp: In function ‘int main()’:
main.cpp:6:38: error: ‘str_view’ is not a member of ‘some_useful_ns’
    6 |         std::cout << some_useful_ns::str_view << std::endl;
      |                                      ^~~~~~~~
main.cpp:7:38: error: ‘chr’ is not a member of ‘some_useful_ns’
    7 |         std::cout << some_useful_ns::chr << std::endl;
      |

I find this strange since it works fine when I use a non const / non constexpr like the lines commented out in test_module.cpp. Also, using the const / constexpr works just as expected when using a traditional implementation without modules.

Anyone have a clue as to why I am unable to get this to work successfully with const / constexpr?
In case it helps, I am using gcc (GCC) 11.2.1 20220127

Thanks in advance.

Upvotes: 2

Views: 2238

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473916

This seems like a compiler bug.

Normally, const-qualified variables have internal linkage, and names with internal linkage cannot be exported. However... there's a specific carveout for such variables which are exported. That is, exported const-qualified variables don't have internal linkage.

Upvotes: 1

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