c++ - "multiple definition of..." error for a full specialisation of a template function -

have project configuration:

./main.cpp   ./type_traints/typetraints.cpp ./type_traints/typetraints.hpp ./type_traints/chapter_20.hpp 

the ./type_traints/cmakelists.txt file is:

 cmake_minimum_required (version 2.8)  add_library(chapter_20 typetraints.cpp) 

and ./cmakelists.txt follows:

cmake_minimum_required (version 2.8) project (mpl)  add_subdirectory(type_traints) include_directories(type_traints) link_directories(type_traints)  add_executable (mpl main.cpp) target_link_libraries(mpl chapter_20) 

relevant parts of files (most includes omitted) include:
./type_traints/chapter_20.hpp

#ifndef chapter_20_guard #define chapter_20_guard #include <typetraints.hpp>  void chapter_20() {   test_23(); }    #endif //chapter_20_guard 

./type_traints/typetraints.hpp

#ifndef type_traints_guard #define type_traints_guard namespace details {    template<class t> const char* class2name() {     return "unknown";   };    template<> const char* class2name<int>() {     return "int";   }; }  template<class t> class type_descriptor {   friend std::ostream& operator << (std::ostream& stream,                                      const type_descriptor<t>& desc) {     stream << desc.getname();     return stream;   }  public:   std::string getname() const;   };  template<class t> std::string type_descriptor<t>::getname() const {   return details::class2name<t>(); }  void test_23();     #endif // type_traints_guard 

./type_traints/typetraints.cpp

#include<typetraints.hpp>  void test_23() {   cout << type_descriptor<int>() << endl; } 

and ./main.cpp

#include <chapter_20.hpp>  int main(int argc, char* argv[]) { chapter_20();   return 0; } 

the project compiles fails link:

[ 50%] building cxx object type_traints/cmakefiles/chapter_20.dir/typetraints.cpp.o linking cxx static library libchapter_20.a [ 50%] built target chapter_20 [100%] building cxx object cmakefiles/mpl.dir/main.cpp.o linking cxx executable mpl type_traints/libchapter_20.a(typetraints.cpp.o): in function `char const* details::cl ass2name<int>()':                                                                    /home/marcin/projects/mpl/type_traints/typetraints.hpp:312: multiple definition of `c har const* details::class2name<int>()'                                               cmakefiles/mpl.dir/main.cpp.o:/home/marcin/projects/mpl/type_traints/typetraints.hpp: 312: first defined here                                                              collect2: ld returned 1 exit status make[2]: *** [mpl] błąd 1 make[1]: *** [cmakefiles/mpl.dir/all] error 2 make: *** [all] error 2 23:56:20@marcin-laptop ~/p 

the project links fine if remove class2name specialization (class2name<int>()) typetraints.hpp , use generic implementation.

does have idea why that? did miss-configure cmake files?

in short: explicitly (i.e. fully) specialized template function is no longer template. ordinary function , obeys 1 definition rule ordinary functions.

in other words can't define explicitly specialized function templates in header file. doing result in odr violations.

a template template long depends on @ least 1 parameter. i.e. partial specializations can defined in header files (since still templates). explicit (i.e. full) specializations can declared in header files, have defined in implementation files, ordinary functions.