CMake failed to link submodule, undefined reference

Question

I don't know why error appears, i use target_link_libraries

c:/dev-tools/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/11.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles\submoduleTest1.dir/objects.a(main.cpp.obj):main.cpp:(.text.startup+0x2c): undefined reference to `void fmt_println2<char [5]>(char const (&) [5])'
collect2.exe: error: ld returned 1 exit status
mingw32-make[3]: *** [CMakeFiles\submoduleTest1.dir\build.make:101: submoduleTest1.exe] Error 1
mingw32-make[2]: *** [CMakeFiles\Makefile2:99: CMakeFiles/submoduleTest1.dir/all] Error 2
mingw32-make[1]: *** [CMakeFiles\Makefile2:106: CMakeFiles/submoduleTest1.dir/rule] Error 2
mingw32-make: *** [Makefile:123: submoduleTest1] Error 2

Main CMake:

cmake_minimum_required(VERSION 3.24)
project(submoduleTest1)


add_subdirectory(fmt_formatter)

set(CMAKE_CXX_STANDARD 17)

add_executable(submoduleTest1 main.cpp)
target_link_libraries(submoduleTest1 fmt_formatter)

fmt_formatter CMake:

find_package(fmt CONFIG REQUIRED)
add_library(fmt_formatter STATIC fmt_formatter.cpp fmt_formatter.h)
target_link_libraries(fmt_formatter fmt::fmt)

fmt_formatter.cpp

#include "fmt_formatter.h"

template<typename... Args>
void fmt_println(const fmt::text_style &ts, bool color, const Args &... args) {
    fmt::print(color ? ts : fmt::text_style(), args...);
    fmt::print("\n");
    fflush(stdout);
}

template<typename... Args>
void fmt_println2(const Args &... args) {
    fmt::print(fmt::text_style(), args...);
    fmt::print("\n");
    fflush(stdout);
}

fmt_formatter.h

#include "fmt/core.h"
#include "fmt/color.h"

template<typename... Args>
void fmt_println(const fmt::text_style &ts, bool color = true, const Args &... args);

template<typename... Args>
void fmt_println2(const Args &... args);

main.cpp

#include <iostream>
#include "fmt_formatter/fmt_formatter.h"

int main() {
    std::cout << "Hello, World!" << std::endl;
    fmt_println2("test");
    return 0;
}

Any ideas how can i solve it? I tried many solutions but any of them worked.

score 1 · Accepted Answer · answered Feb 06 '23 at 00:26

Move your template function definitions to the header file. For a template to be instantiated at compile time, the definition needs to be visible at that point, which it isn't if you put it in a cpp file and not the hpp file that gets included where the instantiation is happening.

For more info, see Why can templates only be implemented in the header file?

CMake failed to link submodule, undefined reference

1 Answers1