About template functions

ahbi82 · #1 29-Jan-2010, 09:26

Template classes has to be coded only in the header files. Does this rule apply to template functions?

One example i can think of is that it is being used as a static function. Meaning its being use by another function define in the header file.

CPP / C++ / C Code:

/* example.hpp */
#ifndef __EXAMPLE_H__
#define __EXAMPLE_H__

int MyFunc(int _arg);

#endif // __EXAMPLE_H__

CPP / C++ / C Code:

#include "example.hpp"

template<typename T>
T square(const T &_arg)
{
    return _arg * _arg;
}

int MyFunc(int _arg)
{
    return square<int>(_arg);
}

If the static function is not being used, will it still be compiled into obj file? I know it sounds stupid, but i think it wouldn't cause T is not being specified.

davekw7x · #2 29-Jan-2010, 11:37

Quote:

Originally Posted by ahbi82

...but i think...

The template of a templated function must be known to the compiler wherever you use it (usually you do this by putting the template definition in a header file).

CPP / C++ / C Code:

// square.h
#ifndef _SQUARE_H__
#define _SQUARE_H__

template < typename T > T square(const T & _arg)
{
    return _arg * _arg;
}

#endif

CPP / C++ / C Code:

// myfunc.h
#ifndef _MYFUNC_H__
#define _MYFUNC_H__

int MyFunc(int);

#endif

CPP / C++ / C Code:

// myfunc.cpp
#include "square.h"
int MyFunc(int _arg)
{
    return square < int >(_arg);
}

CPP / C++ / C Code:

// test2.cpp
#include "myfunc.h"

#include <iostream>
using namespace std;

int main()
{
    int x;
    int y;
    x = 42;
    y = MyFunc(x);
    cout << "x = " << x << ", y = " << y << endl;
    return 0;
}

Compiled with:

Code:

 g++ -c -Wall -W -pedantic myfunc.cpp
 g++ -c -Wall -W -pedantic test2.cpp
 g++ myfunc.o test2.o -o test2

Output is:

Code:

./test2
x = 42, y = 1764

Note that the main() function doesn't know or care that a templated function is used; it only needs to know the signature of MyFunc. Since MyFunc uses the templated function in its implementation, the template must be known to the compiler when myfunc.cpp is compiled.

Regards,

Dave

Footnote: Your use of the word "static" is inappropriate for this discussion. I think you mean "non-templated function" or some such thing. The keyword "static" has a specific meaning unrelated to this topic.

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About template functions

Re: about template functions.