c++ Class template function external error [duplicate]

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• Why can templates only be implemented in the header file? 13 answers

i was making a config file parser and i had a error. i followed this website to make it: http://www.dreamincode.net/forums/topic/183191-create-a-simple-configuration-file-parser/

i tried to separate it into a header and cpp file:

header:

class ConfigFile {

private: //vars and stuffs here 


public:

template <typename ValueType>
ValueType getValueOfKey(const std::string &key, ValueType const &defaultValue) const;
};

cpp:

template <typename ValueType> 
ValueType ConfigFile::getValueOfKey(const std::string &key, ValueType const &defaultValue = ValueType()) const 
{ if (!keyExists(key)) return defaultValue;

return Convert::string_to_T<ValueType>(contents.find(key)->second);
}

another cpp that executes it:

bool configstuff() {

    ConfigFile cfg("spotify.ini");

//  cfg.keyExists("car");

    string someValue = cfg.getValueOfKey<string>("mykey", "Unknown");

//  string carValue = cfg.getValueOfKey<string>("car");

//  MessageBox(0, (LPCWSTR)someValue.c_str(), L"Msg title", MB_OK | MB_ICONQUESTION);





}

the function declaration and definition are ok. the line "ConfigFile cfg("spotify.ini");" is ok too (i commented other lines out to test it).

thats mean the error is in cfg.getValueOfKey<string>("mykey", "Unknown");

i got the error

    Severity    Code    Description Project File    Line    Suppression State
Error   LNK2001 unresolved external symbol "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl ConfigFile::getValueOfKey<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)const " (??$getValueOfKey@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@ConfigFile@@QEBA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@AEBV12@0@Z)

i have no idea where the problem is.

i have tried fixing it for more than 4 hours but stil couldnt find the solution. please help. thanks!