undefined reference to 'std::string Helper::ToString<int>(int const&)'

Question

I get the error:

undefined reference to 'std::string Helper::ToString<int> (int const&)' in Helper.h on line ToString(y):

Helper.h

#ifndef HELPER_H
#define HELPER_H

#include <ctime>
#include <string>
#include <sstream>
#include <fstream>

namespace Helper
{
    template <class T>

    std::string ToString(const T &);

    struct DateTime
    {
        DateTime()
        {
            time_t ms;
            time (&ms);

            struct tm *info = localtime(&ms);

            D = info->tm_mday;
            m = info->tm_mon + 1;
            y = 1900 + info->tm_year;
            M = info->tm_min;
            H = info->tm_hour;
            S = info->tm_sec;
        }

        DateTime(int D, int m, int y, int H, int M, int S) : D(D), m(m), y(y), H(H), M(M), S(S) {}
        DateTime(int D, int m, int y) : D(D), m(m), y(y), H(0), M(0), S(0) {}


    DateTime Now() const
    {
        return DateTime();
    }

    int D, m, y, H, M, S;


    std::string GetDateString() const
    {
        return std::string( D < 10 ? "0" : "") + ToString(D) +
               std::string( m < 10 ? ".0" : ".") + ToString(m) + "." +
               ToString(y);

    }

    std::string GetTimeString(const std::string &sep = ":") const
    {
        return std::string( H < 10 ? "0" : "") + ToString(H) + sep +
               std::string( M < 10 ? "0" : "") + ToString(M) + sep +
               std::string( S < 10 ? sep : "") + ToString(S);
    }

    std::string GetDateTimeString( const std::string &sep = ":") const
    {
        return GetDateString() + " " + GetTimeString(sep);
    }
    };
};
template <class T>

std::string ToString(const T &e)
{
    std::ostringstream s;
s << e;
return s.str();
}

void WriteAppLog( const std::string &s)
{
std::ofstream file ("AppLog.txt", std::ios::app);
file << "[" << Helper::DateTime().GetDateTimeString() << "]" <<
"\n" << s << std::endl << "\n";
file.close();
}


#endif // HELPER_H

KeyConstant.h

#ifndef KEYCONSTANT_H
#define KEYCONSTANT_H

#include <map>
#include <string>

class KeyPair
{
public:
    KeyPair (const std::string &vk = "", const std::string &name = "") : VKName (vk), Name (name) {}

std::string VKName;
std::string Name;
};

class Keys
{
public:
static std::map<int, KeyPair> KEYS;

};
#endif // KEYCONSTANTS_H

main.cpp

#include <iostream>
#include <windows.h>
#include "Helper.h"
#include "KeyConstant.h"
#include "Base64.h"

using namespace std;

int main ()
{
MSG Msg;

while (GetMessage (&Msg, NULL, 0, 0))
{
    TranslateMessage(&Msg);
    DispatchMessage(&Msg);
}
return 0;
}

Answer 1

Bind the implementation to your namespace:

template <class T>
std::string Helper::ToString(const T &e)
         // ^^^^^^^^
{
    std::ostringstream s;
    s << e;
    return s.str();
}

or place it inline:

namespace Helper
{
    template <class T>
    std::string ToString(const T &e) {
         std::ostringstream s;
         s << e;
         return s.str();
    }
    // ...
}

Answer 2

You have declared the method Helper::ToString in the namespace Helper but not defined it anywhere. The definition of the ToString method in Helper.h outside of the namespace knows nothing about the declaration inside the namespace.

Either bind the implementation as the other answer suggests, or define this method inside the Helper namespace itself where you have declared it.

Answer 3

In header file, you need to put this part

template <class T>

std::string ToString(const T &e)
{
    std::ostringstream s;
s << e;
return s.str();
}

void WriteAppLog( const std::string &s)
{
std::ofstream file ("AppLog.txt", std::ios::app);
file << "[" << Helper::DateTime().GetDateTimeString() << "]" <<
"\n" << s << std::endl << "\n";
file.close();
}

into the borders of "namespace Helper". Currently, it is outside. As well, you need to remove a semi column after "namespace Helper"'s ending curly bracket.