Re: template class, methods and friend, unable to link

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 18 Dec 2009 17:20:56 -0500

Message-ID:

<hggv89$7qv$1@news.datemas.de>

suresh.amritapuri wrote:

[..]
I am able to make the code working by including the definitions of the
friend function inside the header file. But Faq 35.16 suggests that it
can be done otherwise, but it didnot work for me. Here is my
"complete" code.

[..]

Let's make it very simple to start.

------------------------------------------------ foo.h
template<class T> void foo(T t); // declaration
------------------------------------------------ foo.cpp
#include <foo.h>
#include <typeinfo>
#include <iostream>

template<class T> void foo(T t) // definition
{
    std::cout << "foo(" << typeid(T).name() << ")\n";
}

template void foo<int>(int); // explicit instantiation
template void foo<double>(int); // explicit instantiation
------------------------------------------------ main.cpp
#include <foo.h>

int main()
{
     foo(42); // OK
     foo(3.14159); // OK
     foo('a'); // will not link : foo<char> undefined
}
----------------------------------------------------------

Now, the code above is untested, but conveys the general idea of how
templates can be made "available" without including the definitions of
the functions into the header: the explicit instantiation mechanism.

V
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