Overloaded Stream Operators with Templates

[42Wired]

I am trying to write an API (called logEvent) for an outdated os-level logging function. I would like to call it in the following manner.

logEvent << "string";
// or
logEvent << "string" << 123 << 0.456 << ... ;

I'm trying to accomplish this by overloading the stream operator, and making it take a template argument so it will pass it to the outdated logging function. I have tried befriending ostream, but I can't get it to work either way, nor do I really understand why I need to do that.

The way I have it right now, I get the following linker error, but only when I call the overloaded operator from main.cpp (below).

main.obj : error LNK2019: unresolved external symbol "public: class logEvent * __thiscall logEvent::operator<<<int>(int)" (??$?6H@logEvent@@QAEPAV0@H@Z) referenced in function _main

I have been up and down the web (including this forum) and have not been able to find a solution. Any help is greatly appreciated.

// main.cpp
#include "logEvent.h"

using namespace std;
int main()
{
    logEvent e;
    e << 123;

    return 0;
}

// logEvent.h
#include <string>
#include <sstream>

class logEvent
{
public:
    logEvent();

    template< typename T >
    logEvent *operator<<( T arg );
}

// logEvent.cpp
#include <string>
#include <sstream>

template< typename T >
logEvent *logEvent::operator<<( T arg )
{
    oldLog( arg );

    return this;
}

[kvprajapati]

Return reference of logEvent object.

#include <string>
#include <sstream>

class logEvent
{
public:
    logEvent();

    template< typename T >
    logEvent &operator<<( T arg );
}

template< typename T >
logEvent &logEvent::operator<<( T arg )
{
    oldLog( arg );
    return *this;
}

[42Wired]

Thanks for replying.

When I make the changes you gave me, I get a linker error:

main.obj : error LNK2019: unresolved external symbol "public: class logEvent & __thiscall logEvent::operator<<<int>(int)" (??$?6H@logEvent@@QAEAAV0@H@Z) referenced in function _main

I created a new project and put in exactly the code I posted in the OP, just to see if the problem was something extraneous, but no luck.

[kvprajapati]

Please post your code. Linker error shows that you didn't implement such a method (post #3).

[42Wired]

Please post your code. Linker error shows that you didn't implement such a method (post #3).

I'm hoping it's something stupid I'm over-looking. I'm using MS Visual Studio 2005 to compile, btw.

// main.cpp
#include <iostream>
#include "logEvent.h"

using namespace std;

int main()
{
    logEvent e;

    cout << "\n                   ***      LOGIC TEST FOR logEvent.cpp      ***\n" << endl;

    e << 123;

    return 0;
}

// logEvent.h
#ifndef LOGEVENT_H
#define LOGEVENT_H

#include <string>
#include <sstream>

class logEvent
{
public:
    logEvent();

    template< typename T >
    logEvent &operator<<( T arg );
};

#endif // LOGEVENT_H

// logEvent.cpp

#include "logEvent.h"

#include <string>
#include <sstream>

//#define TERMINAL_WINDOW_WIDTH   80

using namespace std;

logEvent::logEvent()
{
    
}

template< typename T >
logEvent &logEvent::operator<<( T arg )
{
    cout << arg << endl;
    return *this;
}

[Dave Sinkula]

http://www.parashift.com/c++-faq-lite/templates.html#faq-35.13 and thereabouts.

[42Wired]

http://www.parashift.com/c++-faq-lite/templates.html#faq-35.13 and thereabouts.

This is good information, and it offered three solutions:

1. Move the template function definition into the .h file
2. Include a function definition in the .cpp file
3. Create a third file, such as logEvent-impl.cpp, containg only the following code:

   #include "logEvent.cpp"
    
   template logEvent &operator<<<T>( T arg );

None of these seemed to work. The errors I received, respectively, are as follows:

1. 1>main.obj : error LNK2019: unresolved external symbol "public: class logEvent & __thiscall logEvent::operator<<<int>(int)" (??$?6H@logEvent@@QAEAAV0@H@Z) referenced in function _main
2. I can't declare the function outside of the class definition, so the error message is irrelevant.
3. 1>c:\documents and settings\usr\my documents\visual studio 2005\projects\logevent\logevent\logeventstreamop.cpp(1) : fatal error C1083: Cannot open include file: 'logEvent.cpp': No such file or directory (And I did remember to add the new file to the project.)

I read almost that entire page, and these were the only solutions it suggested. Defining a separate method for each instance will cause me to have to write three more methods for those methods, so I would REALLY like to be able to use a template here. Why, oh, why can't we just use Java here?

[Dave Sinkula]

The first suggestion worked dandy for me, starting with the code you posted. I'd recommend trying the suggested remedies again.

[42Wired]

The first suggestion worked dandy for me, starting with the code you posted. I'd recommend trying the suggested remedies again.

I tried it again, and now the first one works fine... I must have not changed it properly. I don't know if it's going to be acceptable to define the function in the .h file, but they'll have to live with it. Thanks to everyone for all their help!