Re: (what ought to be a) simple template question

From:

ymett <ymett.on.usenet@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 22 Apr 2009 18:28:38 CST

Message-ID:

<44bc9db2-dee6-4113-b2fd-b7edf2b2fe8d@v15g2000yqn.googlegroups.com>

On Apr 16, 7:25 am, Ian <ianm...@comcast.net> wrote:

Hi! Here is something that has been driving me crazy. Imagine the following
header and two cpp files:

// file.h
#include <stdio.h>

template<class T>
class X
{
public:
   void nonInlineFunction() const;

};

template<class T>
void X<T>::nonInlineFunction() const
{
   for( int i = 0; i < 2; i++ ); // here only to make it noninline
   printf( "Generic\n" );

}

void f();

// file1.cpp
#include "file.h"

template<>
void X<int>::nonInlineFunction() const
{
   for( int i = 0; i < 2; i++ ); // here only to make it noninline
   printf( "Specialized for int\n" );

}

void f()
{
   X<int> x;
   x.nonInlineFunction();

}

// file2.cpp
#include "file.h"

int main()
{
   X<int> x;
   x.nonInlineFunction();
   f();
   return 0;

}

When I compile and run these, I get the following output

(a) g++ 2.81:
        Generic
        Specialized for int

(b) g++ 2.91.66
        Specialized for int
        Specialized for int

These two are very old; don't expect to get standard-conforming
behaviour from them.

(c) Visual C++
Does not link, the error is

file1.obj : error LNK2005: "public: void __thiscall
X<int>::nonInlineFunction(void)const " (?nonInlineFunction@?$X@H@@QBEXXZ)
already defined in file2.obj

You don't say what version of VC++, but it doesn't matter; the error
message is perfectly correct.

You need to explicitly specialise the class for int before you can
define the member functions.

Add the following to the header:

template<>
class X<int>
{
public:
    void nonInlineFunction() const;
};

Note that the other responses are incorrect on this point -- you can't
specialise a single (non-template!) member function of a template
class.

Yechezkel Mett

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