Help with templates

Hi!! i'm using templates for the first time and trying to implement it in one the programs which had many constructors taking different types to convert it into a string. I've got most of the part correct and also I read on one of the forums that the constructor and other member functions should be declared in the header file and not in the .cpp file.
Now the problem is I have a static function that returns a string. The code in .cpp file is:

template<class T>
string CXMLString<T>::m_error[_XMLMAX] =
{ "XMLFULLLINE", "XMLOPENTAG", "XMLCLOSETAG", "XMLERRORSYNTAX",
		"XMLERRORTAGMISMATCH" };

template<class T>
string CXMLString<T>::error(XMLParseResult_t err)
{
	return m_error[err];
}

but if i'm calling this method from main() i'm getting an error saying undefined address. How can I implement this in the header file itself since I cannot initialize the array within the class?

closed account (o3hC5Di1)

Hi there,

Template functions / classes need to be defined in the header files.
This explains why: http://www.parashift.com/c++-faq-lite/templates-defn-vs-decl.html
It also links to a few possible alternative solutions.

Hope that helps.

All the best,
NwN

noisycoder (58)

Well I understand that, but if I'm choosing the first option of keeping it all in the header file how do i initialize the member attributes of the class. Also, trying the second method and keeping the method definition in another .cpp file like:

#include <iostream>
#include <string>
#include <stdlib.h>
#include <sstream>
#include "CXMLStringTemplate.h"

using namespace std;
using namespace CXML;

template<class T>
string CXMLString<T>::m_error[_XMLMAX] =
{ "XMLFULLLINE", "XMLOPENTAG", "XMLCLOSETAG", "XMLERRORSYNTAX",
		"XMLERRORTAGMISMATCH" };

template<class T>
string CXMLString<T>::error(XMLParseResult_t err)
{
	return m_error[err];
}

template string error<string>(XMLParseResult_t err);

throws an error: symbol error could not be resolved

cire (8284)

TemplateClass.h:

#ifndef TEMPLATECLASS_H_
#define TEMPLATECLASS_H_

#include <string>

template <typename T>
class C
{
public:
    enum ErrorType { One, Two, Three } ;

    static std::string error(ErrorType err) ;

private:
    static std::string m_error[3] ;

};


template <typename T>
std::string C<T>::error( typename C<T>::ErrorType err )
{
    return m_error[err] ;
}

template <typename T>
std::string C<T>::m_error[3] = {"1", "2", "3" };

#endif

Main.cpp:

#include <string>
#include <iostream>
#include "templateClass.h"

int main()
{
    std::cout << C<int>::error(C<int>::One) << '\n' ;
}

noisycoder (58)

Hi! thanks for that code. However in the mean while what I tried was doing a #include of the .cpp file itself in my main() since I thought its just a linking error and not anything to do with compilation. Now it is working fine, but just want to know is it legit to do this. Is it a good programming technique or is the bit of code given by you a better solution?

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