Thread: Template functions

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