Template function specialization once again (test a little, post a little)

Newsgroup: comp.lang.c++

Subject: Template function specialization once again (test a little, post a little)

From: Daniel <danielaparker@...>

Date: Mon, 8 Jul 2013 19:26:04 -0700 (PDT)



Special thanks to everyone who attempted to responded to my

original post, particularly Geoff and Gerhard.



This time I've hopefully posted code that better illustrates

my question. Basically the question is, does the output from

VC 10 for examples 1 and 2 below conform to expected output?



The context is a C++ template library that supports a default

serialization method for custom types, but allows the

library user to override it with something more sensible.



Example 1

=========



test.h

------



#ifndef TEST_HPP

#define TEST_HPP



#include <iostream>



namespace ns

{

template <class T>

void serialize(std::ostream& os, const T& A)

{

os << "null" << std::endl;

}



template <class T>

class A

{

public:

A(const T& A)

: A_(A)

{

}

void serialize(std::ostream& os) const

{

ns::serialize(os,A_);

}



T A_;

};

}

#endif



test1.h

-------



#ifndef TEST1_HPP

#define TEST1_HPP



void f();



#endif



test.cpp

--------



#include "test.h"

#include "test1.h"



int main()

{

ns::A<double> a(10.0);

a.serialize(std::cout);

f();

}





test1.cpp

---------



#include "test.h"



void f()

{

ns::A<double> b(10.0);

b.serialize(std::cout);

}



Visual Studio 10 gives the linker error



>test1.obj : error LNK2005: "void __cdecl ns::serialize etc etc



: fatal error LNK1169: one or more multiply defined symbols found



So, test.cpp and test1.cpp induce different instantiations of

the function serialize with the same arguments?



Example 2

=========



test.h

------



Same as above



test1.h

-------



#ifndef TEST1_HPP

#define TEST1_HPP



#include <iostream>



void f();



namespace ns

{

template <>

void serialize(std::ostream& os, const double& A)

{

os << A << std::endl;

}

}



#endif



test.cpp

--------



Same as above



test1.cpp

---------



#include "test.h"

#include "test1.h"



void f()

{

ns::A<double> b(10.0);

b.serialize(std::cout);

}



Visual Studio 10 still gives the linker error



test1.obj : error LNK2005: "void __cdecl ns::serialize<double> etc.

LNK1169: one or more multiply defined symbols found



Even with template <>, the linker thinks there are multiple

definitions of serialize per header inclusion?



Example 3

=========



As above, except



test1.h

-------



#ifndef TEST1_HPP

#define TEST1_HPP



#include <iostream>



namespace ns

{

template <> inline

void serialize(std::ostream& os, const double& A)

{

os << A << std::endl;

}

}



#endif



--------------------------------



This time with inline in the template definition

Visual Studio 10 links and gives the correct result.

But should inline be necessary?



Example 4

=========



As above, except



test1.h

-------



#ifndef TEST1_HPP

#define TEST1_HPP



#include <iostream>



namespace ns

{

template <>

void serialize(std::ostream& os, const double& A);

}

#endif



test1.cpp

---------



#include "test.h"

#include "test1.h"



void f()

{

ns::A<double> b(10.0);

b.serialize(std::cout);

}



namespace ns

{

template<>

void serialize(std::ostream& os, const double& A)

{

os << A << std::endl;

}

}

--------------------



Again Visual Studio 10 links and gives the correct result.

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