Template & Arays

**kernel** · 06-07-2004

Hello
i'm trying to solve an exersice i've been given , would appreciate help

i have to creat a class with two templates
and in the main make an array of that class but it keeps poping the error 2955
the exersice says i can't overload :

perator[]
any ideas ?
here's what i've for so far

//Pair.h
template<class T,class S>
class Pair
{

protected:

T rison;
S sheni;

public:
Pair(T mfirst,S msecond);
virtual ~Pair();
T first(){return rison;};
S second(){return sheni;};

};

//Pair.cpp

template<class T,class S>
Pair<T,S>::Pair(T mfirst,S msecond)
{
this->rison=mfirst;
this->sheni=msecond;

cout <<"Pair has been created "<<endl;

}

template<class T,class S>
Pair<T,S>::~Pair()
{
cout <<"Pair has been deleted "<<endl;

}

//main

#include "Pair.h"
#include "Find.h"
const int SIZE=5;

void main()
{

double n1;
char* n2;
Pair* arr;
arr = new Pair [SIZE];

for (int i=0;i<SIZE<i++)
{
cout <<"Enter two values "<<endl;
cout <<"----->";
cin >>n1;
n2 = new char[SIZE];
cin >>n2;
arr[i]->Pair(n1,n2);
}

delete [] arr;
delete [] n2;

}

Now I know I can do this by declaring
in the class Pair
T* rishon
S* sheni
and overloading the operator[]
but i need to decalre the Pair* arr in the main and somehow
create it
Anyone ?

thanks in advance

**Prelude** · 06-07-2004

You have plenty of problems with your code. Any templated class must be given the template arguments when you use it, so any use of Pair in your code must be changed to Pair<double, char *>. Before you can create an array of class objects, there must be a default constructor. And lastly, main returns int, not void.

Also, passing char * as a template argument isn't the wisest thing to do because you'll be working with a reference to the memory, not a copy. Any internal state of an object should usually have its own copies of the data. In this case the C++ string class would be much safer.

Code:

#include <iostream>
#include <string>

using namespace std;

template<class T,class S>
class Pair 
{
protected:
  T rison;
  S sheni;
public:
  Pair(): rison(T()), sheni(S()){}
  Pair(T mfirst,S msecond);
  virtual ~Pair();
  T first(){return rison;};
  S second(){return sheni;};
};

template<class T,class S>
Pair<T,S>::Pair(T mfirst,S msecond)
{
  this->rison=mfirst;
  this->sheni=msecond;

  cout <<"Pair has been created "<<endl;
}

template<class T,class S>
Pair<T,S>::~Pair()
{
  cout <<"Pair has been deleted "<<endl;
}

const int SIZE=5;

int main()
{
  double n1;
  string n2;
  Pair<double, string>* arr;
  arr = new Pair<double, string>[SIZE];

  for (int i=0;i<SIZE;i++)
  {
    cout <<"Enter two values "<<endl;
    cout <<"----->";
    cin >>n1;
    cin >>n2;
    arr[i] = Pair<double, string>(n1,n2);
  }
  delete [] arr;
}

**kernel** · 06-07-2004

thanks for the reply
one more question do i have to use
Pair<double, string>* arr
that kinda spoils the idea of templaete if i delcare <double,string>
the idea is that each pair will be decided according to input
sometime double,int sometimes string,string whatever the user chooses ...
can it be done ?

**Prelude** · 06-07-2004

>that kinda spoils the idea of templaete if i delcare <double,string>
How so? You can still use the template class for any combination of valid types. That sounds like an accurate portrayal of parameterized types to me.

>the idea is that each pair will be decided according to input
It doesn't quite work like that. You can specify default template arguments that can be used like this:

Code:

template<class T = double,class S = string>
class Pair {
  ...
}

const int SIZE = 5;

int
main()
{
  Pair<> *arr = new Pair<>[SIZE];
}

But if the client wants anything other than a (double, string) pair, they have to say so explicitly.

>can it be done ?
Only with function templates:

Code:

#include <iostream>
#include <string>

using namespace std;

template <typename T>
void foo(T& arg);

int
main()
{
  int    i = 10;
  double d = 123.456;
  char   c[] = "testing";
  string s("one...two...three");

  foo(i);
  foo(d);
  foo(c);
  foo(s);
}

template <typename T>
void
foo(
  T& arg
  )
{
  cout<< arg <<endl;
}

**kernel** · 06-07-2004

ok , understood

and thanks for the fast and accurate replies
godblles you

(and Tayshaun Prince)

**kernel** · 06-07-2004

Ok this is what i got so far I can't figure out what's wrong
i've searched everywhere

Code:

//MAIN


#include "Pair.h"
#include "Find.h"
const int SIZE=5;


template <typename M>
M send(M& arg);
void main()
{


	double n1;
	int   n2;
	Pair<> *arr = new Pair<>[SIZE];

	for (int i=0;i < SIZE; i++)
	{
		cout <<"Enter two values  "<<endl;
		cout <<"----->";
		cin >>n1;
		cin >>n2;
		arr[i] = Pair<>(send(n1),send(n2));
	}

		
	delete [] arr;
	


}

template <typename M>
M send(M& arg)
{
  return arg;
}




//PAIR.H

template<class T = double,class S = int>
class Pair  
{

protected:
	
	T rison;
	S sheni;
	
public:
	
	Pair(): rison(T()), sheni(S()){};
	Pair(T mfirst,S msecond);
	virtual ~Pair();
	T first(){return rison;};
	S second(){return sheni;};



};

//PAIR.CPP

template<class T,class S>
Pair<T,S>::Pair(T mfirst,S msecond)
{
	this->rison=mfirst;
	this->sheni=msecond;
	
	cout <<"Pair has been created "<<endl;
}


template<class T,class S>
Pair<T,S>::~Pair()
{
	cout <<"Pair has been deleted "<<endl;
}

And this is the error message

main.obj : error LNK2001: unresolved external symbol "public: __thiscall Pair<double,int>::Pair<double,int>(double,int)" (??0?$Pair@NH@@QAE@NH@Z)
main.obj : error LNK2001: unresolved external symbol "public: virtual __thiscall Pair<double,int>::~Pair<double,int>(void)" (??1?$Pair@NH@@UAE@XZ)
Debug/ex2tr2.exe : fatal error LNK1120: 2 unresolved externals
Error executing link.exe.

ex2tr2.exe - 3 error(s), 0 warning(s)

Anyone ?

**jlou** · 06-07-2004

Everything that is in Pair.cpp should be put into Pair.h instead. All or nearly all compilers don't support template class method definitions in the source file, they must be in the header file that is included elsewhere.

**kernel** · 06-08-2004

10x that helped , would it work properly on a borland compiler ?

Now i have another problem ...
it won't override the default values of <class t = double,class S=int>
which sort of makes the template abit unusefull

**Prelude** · 06-09-2004

>it won't override the default values of <class t = double,class S=int>
Are you specifying different template parameters?

Code:

#include <string>

using namespace std;

template<class T = double,class S = string>
class Pair {
};

const int SIZE = 5;

int
main()
{
  // Use the default
  Pair<> *arr = new Pair<>[SIZE];
  // Change the default
  Pair<int, int> *arr2 = new Pair<int, int>[SIZE];
}

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