Template functions

Is it possible to write a function that returns a variable type? My guess would have been to use template functions, although the rest of my program is in C (I have a mixed C/C++ programming environment so I can still use C++ in parts). The following (unrelated) code snippet works for example:

Code:

template <typename T> void Swap(T *Ptr_A,T *Ptr_B)
{
	// Declare variables
	T Temp;

	// Perform swap
	Temp=*Ptr_A;
	*Ptr_A=*Ptr_B;
	*Ptr_B=Temp;
}

but when I try something like this it doesn't work:

Code:

template <typename T> T Comb_C(int M,int R)
{
	// Declare variables
	T Answer=1;

	// Work out mCr
	for(int i=1;i<=R;i++)
	{
		Answer*=(M+1)-i;
		Answer/=i;
	}

	// Return answer
	return Answer;
}

The compiler tells me:

error LNK2019: unresolved external symbol "int __cdecl Comb_C<int>(int,int)" (??$Comb_C@H@@YAHHH@Z) referenced in function _main

when I do

Code:

Comb_C<int>(a,b)

I have declared the function, in exactly the same way I had declared the swap function, so I must assume that that isn't the problem.
I do realise that something like this would work IF I only need the different types to be different types of int (int8,int16,int32,int64) OR diff. types of floating point (float,double):

Code:

#define COMB_C(T,M,R)          ((T) Comb_C((M),(R)))
...
int main()
{
    ...
    printf("%d\n",COMB_C(__int32,12,4));
    ...
}
...
__int64 Comb_C(int M,int R)
{
    __int64 Answer=1;
    for(int i=1;i<=R;i++)
    {
        Answer*=(M+1)-i;
        Answer/=i;
    }
    return Answer;
}

Is there a way to do it with template functions though, where I could theoretically have the output as an integer or double or whatever?

Thx,
Philipp

This should be in the C++ section, not the C section.

Your templated code should work OK. If the linker can't find the function there may be typos or problems with how your project is set up.

Could you post a complete example that produces this error?

For example

Code:

#include <iostream>

template <typename T>
T foo(int n);


int main()
{
    char c = foo<char>(100);
    int i = foo<int>(1278);
    unsigned f = foo<unsigned>(-1);
    std::cout << c << " " << i << " " << f << '\n';
}

template <typename T>
int foo(int n)
{
    return T(n);
}

Aren't you sure that the second red one (int) shouldn't be T?
Anyway, I've been experimenting around with what works and what doesn't and I have put together this program which demonstrates that. Note though that this code excerpt deals simply with template functions and not with the special case of template function for which I asked for help. As it turned out, the other template functions didnt work either. How can I fix it up this code so that the template functions Swap_B and Swap_C work properly?

Code:

// C Board Ex.cpp : Defines the entry point for the console application.
// Programming Environment: Microsoft Visual Studio 6.0 (Visual C++ in this case)

// Includes
#include "stdafx.h" // Contains statements: #pragma once, #include <iostream> and #include <tchar.h>
#include "Swap.h"

// Functions
template <typename T> void Swap_A(T *Ptr_A,T *Ptr_B);

// Main function
int main(void)
{
	// Declare variables
	int a,b;
	double A=1.1,B=2.2;

	// Test functions
	Other_Function(&a,&b); // Fine: Was declared in header at file scope and so extern was unnecessary. Assigns values 1 to a and 2 to b.
	printf("a=%d b=%d A=%3f B=%f\n",a,b,A,B);
	Swap_A<int>(&a,&b); // Fine: Local function which proves that template functions work in my programming environment. Swaps values of a and b
	Swap_A<double>(&A,&B); // Fine: Ditto. Swaps values of A and B
	printf("a=%d b=%d A=%3f B=%f\n",a,b,A,B);
	Swap_B<int>(&a,&b); // Fine: <int> version of Swap_B was specifically called in Swap.cpp in Other_Function() and so no error occurs. Why?? If that function call is commented then this comes up as an error too.
	Swap_B<double>(&A,&B); // Error LNK2019: unresolved external symbol "void __cdecl Swap_B<double>(double *,double *)" (??$Swap_B@N@@YAXPAN0@Z) referenced in function _main
	printf("a=%d b=%d A=%3f B=%f\n",a,b,A,B);
	Swap_C<int>(&a,&b); // Error LNK2019: unresolved external symbol "void __cdecl Swap_C<int>(int *,int *)" (??$Swap_C@H@@YAXPAH0@Z) referenced in function _main
	Swap_C<double>(&A,&B); // Error LNK2019: unresolved external symbol "void __cdecl Swap_C<double>(double *,double *)" (??$Swap_C@N@@YAXPAN0@Z) referenced in function _main
	printf("a=%d b=%d A=%3f B=%f\n",a,b,A,B);

	// Delay exit
	printf("Press enter to exit!");
	fgetc(stdin);

	// Return a value
	return 0;
}

// Swap function
template <typename T> void Swap_A(T *Ptr_A,T *Ptr_B)
{
	// Perform swap
	T Temp=*Ptr_A;
	*Ptr_A=*Ptr_B;
	*Ptr_B=Temp;
}
// EOF



// Swap.cpp : Contains Swap_B function
//

// Includes
#include "stdafx.h"
#include "Swap.h"

// Functions
template <typename T> void Swap_B(T *Ptr_A,T *Ptr_B)
{
	// Perform swap
	T Temp=*Ptr_A;
	*Ptr_A=*Ptr_B;
	*Ptr_B=Temp;
}
template <typename T> void Swap_C(T *Ptr_A,T *Ptr_B)
{
	// Perform swap
	T Temp=*Ptr_A;
	*Ptr_A=*Ptr_B;
	*Ptr_B=Temp;
}
void Other_Function(int *a,int *b)
{
	*a=2;
	*b=1;
	Swap_B<int>(a,b); // Call Swap_B function but not Swap_C function to see the difference that makes
}
// EOF



// Swap.h : Declares Swap_B function
//

// Ensure that code is only executed once
#ifndef SWAP_H
#define SWAP_H

// Functions
template <typename T> extern void Swap_B(T *Ptr_A,T *Ptr_B);
template <typename T> extern void Swap_C(T *Ptr_A,T *Ptr_B);
void Other_Function(int *a,int *b);

// Endif
#endif /*SWAP_H*/
// EOF

I think that what i am doing is aptly described by the comments. I coloured the important parts to make them easier to find. If there is anything else that you need to know, please just ask. I can't explain what's going on...

Thx,
Philipp

I realise now that ur code was supposed to produce an error so forget my very first sentence! (only) lol!

I suggest that you read:
How can I avoid linker errors with my template functions?

Thank you. That helped a lot.

Philipp