Problem with defining Template Function

[SMuzzlim4]

Hello,
I receive the compiler error

Card.cpp:13: undefined reference to `std::string to_String<int>(int const&)'

from this code:

Object.h:

class Object {
public:
	Object();
	virtual ~Object();

	virtual const string toString() const;
};

ostream& operator <<(ostream &, const Object&);

template <class T>
string to_String(const T&);

Card.h:

#include "Object.h"

class Card : public Object {
protected:
	int value;
public:
	Card(int);
	virtual ~Card();

	virtual string toString();
};

Card.cpp:

#include "Card.h"

Card::Card(int value) {
	value = value;
}

Card::~Card() {
}

string Card::toString() {
	return to_String(value);
}

It seems as if the function call to_String(value) doesn't match with to_String(const T&), but I do not see how. Thanks for any help.

[firedraco]

There is no code for that function. You have to supply it in the same file the template is in.

Also, your Card::Card(int) constructor does not do what you think it does. I would rename the parameter to something different.

[SMuzzlim4]

I've defined it in a separate file here:

Object.cpp

#include "Object.h"
#include <sstream>

template <typename T>
string to_String(const T& object) {
	stringstream string_stream;
	string_stream << object;
	return string_stream.str();
}

Is it ok for it to be defined there, or must it be in Object.h?

Thanks for the tip about the constructor. I'm a Java programmer, so I thought that would be correct.

[firedraco]

You can't separate template functions like that unfortunately. It has to be in the in same file as it is used in (e.g. it needs to be the .h file).

[SMuzzlim4]

Does this only apply for templates, or are there also other special functions that must be declared this way?

[firedraco]

Just templates I believe.

[jsmith]

BTW, to_String() is equivalent to boost::lexical_cast<std::string>(). You might consider using it
instead of reinventing. (<boost/lexical_cast.hpp>, if you have it installed.)