What is the bug in the following project?

main.cpp

#include "template_specialization_conflict_test.hpp"

int main()
{
    std::cout << utils::my_template_function(0.555);
    std::cout<<utils::my_template_function<double>(0.555);
    return 0;
}

template_specialization_conflict_test.hpp

#ifndef UTILS__UTILS__UTILS__UTILS
#define UTILS__UTILS__UTILS__UTILS
#include <iostream>

namespace utils
{
    // A generic function
    template <class T>
    T my_template_function(T parameter)
    {
        std::cout << "function template";
        std::cout << parameter;
        return parameter;
    }

    // Template Specialization
    //      A function specialized for double data type
    template <>
    double my_template_function<double>(double parameter)
    {
        std::cout << "function specialization on double";
        std::cout << parameter;
        return parameter;
    }
}
#endif

template_specialization_conflict_test.cpp

#include "template_specialization_conflict_test.hpp"

namespace utils
{
    //empty
}

Error

>------ Rebuild All started: Project: template_specialization_conflict_test, Configuration: x64-Debug ------
  [1/1] Cleaning all built files...
  Cleaning... 2 files.
  [1/3] Building CXX object CMakeFiles\template_specialization_conflict_test.dir\main.cpp.obj
  [2/3] Building CXX object CMakeFiles\template_specialization_conflict_test.dir\template_specialization_conflict_test.cpp.obj
  [3/3] Linking CXX executable template_specialization_conflict_test.exe
  FAILED: template_specialization_conflict_test.exe 
  cmd.exe /C "cd . && "C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" -E vs_link_exe --intdir=CMakeFiles\template_specialization_conflict_test.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\10.0.17763.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\10.0.17763.0\x64\mt.exe --manifests  -- "C:\PROGRA~2\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\bin\Hostx64\x64\link.exe" /nologo CMakeFiles\template_specialization_conflict_test.dir\template_specialization_conflict_test.cpp.obj CMakeFiles\template_specialization_conflict_test.dir\main.cpp.obj  /out:template_specialization_conflict_test.exe /implib:template_specialization_conflict_test.lib /pdb:template_specialization_conflict_test.pdb /version:0.0 /machine:x64 /debug /INCREMENTAL /subsystem:console  kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cd ."
  LINK Pass 1: command "C:\PROGRA~2\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\bin\Hostx64\x64\link.exe /nologo CMakeFiles\template_specialization_conflict_test.dir\template_specialization_conflict_test.cpp.obj CMakeFiles\template_specialization_conflict_test.dir\main.cpp.obj /out:template_specialization_conflict_test.exe /implib:template_specialization_conflict_test.lib /pdb:template_specialization_conflict_test.pdb /version:0.0 /machine:x64 /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:CMakeFiles\template_specialization_conflict_test.dir/intermediate.manifest CMakeFiles\template_specialization_conflict_test.dir/manifest.res" failed (exit code 1169) with the following output:
C:\Users\pc\source\repos\template_specialization_conflict_test\out\build\x64-Debug\main.cpp.obj : error LNK2005: "double __cdecl utils::my_template_function<double>(double)" (??$my_template_function@N@utils@@YANN@Z) already defined in template_specialization_conflict_test.cpp.obj
C:\Users\pc\source\repos\template_specialization_conflict_test\out\build\x64-Debug\template_specialization_conflict_test.exe : fatal error LNK1169: one or more multiply defined symbols found
  ninja: build stopped: subcommand failed.

Rebuild All failed.

How can I fix this?

CodePudding user response：

How can I fix this?

You could solve this by adding/using the keyword inline for the specialization so the specialization would look like:

   //note the keyword inline in the below specialization
   template <> inline
    double my_template_function<double>(double parameter)
    {
        std::cout << "function specialization on double";
        std::cout << parameter;
        return parameter;
    }

This works as can be seen here.

Second way to solve this would be to move your specialization into the source file instead of the header. So your template_specialization_conflict_test.cpp would look like: template_specialization_conflict_test.cpp

#include "template_specialization_conflict_test.hpp"

namespace utils
{
    // Template Specialization
    //      A function specialized for double data type
    template <> 
    double my_template_function<double>(double parameter)
    {
        std::cout << "function specialization on double";
        std::cout << parameter;
        return parameter;
    }
}

The program works as can be seen here and here(with gcc and clang).