Calling template function, I get an error

I got this error;
LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl StrUtils::ToString<int>(int)" (??$ToString@H@StrUtils@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@H@Z) referenced in function "private: void __thiscall CRuntime::CreateXMLMapFile(wchar_t const *)" (?CreateXMLMapFile@CRuntime@@AAEXPB_W@Z)

In my definition files

"StrUtils.cpp"

#include "stdafx.h"
#include <stdlib.h>
#include <assert.h>
#include <sstream>      // std::istringstream


namespace StrUtils {

template<typename T> string ToString(T val)
	{
		static std::ostringstream ostr; //output string stream
		ostr.str(std::string());
		ostr.clear();
		ostr<<val;
		return ostr.str();
		//string abc;
		//return abc;
	}
}

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"StrUtils.hpp"

namespace StrUtils {
	template<typename T>  string ToString(T val);
}

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In my calling program,

#include "StrUtils.h"
...
	std::cout <<StrUtils::ToString<int>(123);

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Can someone let me know why I get the link error