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simple OOP Linker error help

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Started by sheep19 May 08, 2008 08:07 AM

14 comments, last by sheep19 15 years, 11 months ago

sheep19

494

Author

May 08, 2008 08:07 AM

Hello. It's been a while since I even coded because school exams are approaching, so I decided to make a quick revision today. I grabbed my book and wrote a simple program. I decided to break the code in files myself. So, I have done it, but I'm missing something because I get a nasty liker error (my worst). So here's my code: Classes.h


#ifndef CLASSES_H
#define CLASSES_H

class Critter
{
public:
	Critter( const std::string& name = "" );
	std::string GetName() const;

private:
	std::string m_Name;
};

class Farm
{
public:
	Farm( int spaces = 1 );
	void Add( const Critter& aCritter );
	void RollCall() const;

private:
	std::vector< Critter > m_Critters;
};

#endif

main


#include <iostream>
#include <string>
#include <vector>

#include "Classes.h"

int main()
{
	Critter crit( "Poochie" );
	std::cout << "My critter's name is " << crit.GetName() << std::endl;

	std::cout << "\nCreating critter farm.\n";
	Farm myFarm(3);

	std::cout << "\nAdding three critters to the farm.\n";
	myFarm.Add( Critter("Moe") );
	myFarm.Add( Critter("Larry") );
	myFarm.Add( Critter("Curly") );

	std::cout << "\nCalling Roll...\n";
	myFarm.RollCall();

	system("PAUSE");
	return 0;
}

Classes.cpp


#include <iostream>
#include <string>
#include <vector>

#include "Classes.h"

Critter::Critter( const std::string& name ): m_Name( name )
{
}

inline std::string Critter::GetName() const
{
	return m_Name;
}

Farm::Farm( int spaces )
{
	m_Critters.reserve( spaces );
}

void Farm::RollCall() const
{
	for( std::vector< Critter >::const_iterator iter = m_Critters.begin(); iter < m_Critters.end(); ++iter )
		std::cout << iter->GetName() << " here.\n";
}

Edit: Oops, forgot the error. 1>Linking... 1>main.obj : error LNK2019: unresolved external symbol "public: void __thiscall Farm::Add(class Critter const &)" (?Add@Farm@@QAEXABVCritter@@@Z) referenced in function _main 1>C:\My Documents\Visual Studio 2005\Projects\Test2\Debug\Test2.exe : fatal error LNK1120: 1 unresolved externals [Edited by - sheep19 on May 8, 2008 8:36:20 AM]

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MJP

20,295

May 08, 2008 08:11 AM

What's the linker error?

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gah_ribaldi

124

May 08, 2008 08:30 AM

Shouldn't there be a classes.cpp file with the actual implememtation of the class methods?

sheep19

494

Author

May 08, 2008 08:35 AM

Quote:Original post by gah_ribaldi
Shouldn't there be a classes.cpp file with the actual implememtation of the class methods?

Yes, there is, but I haven't included it. Maybe I should.

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shadowcomplex

445

May 08, 2008 08:46 AM

Um, where's your Add( ... ) function for farm?

You declare it in classes.h, but in your classes.cpp there is no implementation. So of course, when you try to invoke that function in main, your link will fail.

Just looks like you forgot to write the actual function :)

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Cygon

1,219

May 08, 2008 08:48 AM

Well, that should be pretty straight forward. Just what the linker complains about: You didn't provide an implementation of the Farm::Add() method.

This is what's missing from your Classes.cpp file:

void Farm::Add(const Critter& aCritter) {  m_Critters.push_back(aCritter);}

EDIT: Darn, beaten to it. Need... to... type... faster!

-Markus-

Professional C++ and .NET developer trying to break into indie game development.
Follow my progress: http://blog.nuclex-games.com/ or Twitter - Topics: Ogre3D, Blender, game architecture tips & code snippets.

sheep19

494

Author

May 08, 2008 09:13 AM

What a silly mistake! :(

Thanks for the help!

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sheep19

494

Author

May 11, 2008 04:52 AM

I decided to split the classes in different files. I achieved this in two ways, but I'm not sure which one is the best.

1ST WAY:

Critter.h

#ifndef CRITTER_H#define CRITTER_H<b>class Farm;</b>class Critter{public:	Critter( const std::string& name = "" );	std::string GetName() const;private:	std::string m_Name;};#endif

Farm.h

#ifndef FARM_H#define FARM_H    //#include "Critter.h" -> not using thisclass Critter;class Farm{public:	Farm( int spaces = 1 );	void Add( const Critter& aCritter );	void RollCall() const;private:	std::vector< Critter > m_Critters;};#endif

Farm.cpp

#include <iostream>#include <string>#include <vector>#include "Farm.h"#include "Critter.h" // ->have to use thisFarm::Farm( int spaces ){	m_Critters.reserve( spaces );}void Farm::Add( const Critter& aCritter ){	m_Critters.push_back( aCritter );}void Farm::RollCall() const{	for( std::vector< Critter >::const_iterator iter = m_Critters.begin(); iter < m_Critters.end(); ++iter )		std::cout << iter->GetName() << " here.\n";}

So I'm avoiding cyclic dependency. I think it's unnecessary though because Critter doesn't need Farm.

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

2nd way

Critter.h

#ifndef CRITTER_H#define CRITTER_Hclass Critter{public:	Critter( const std::string& name = "" );	std::string GetName() const;private:	std::string m_Name;};#endif

Farm.h"

#ifndef FARM_H#define FARM_H#include "Critter.h"class Farm{public:	Farm( int spaces = 1 );	void Add( const Critter& aCritter );	void RollCall() const;private:	std::vector< Critter > m_Critters;};#endif

Farm.cpp

#include <iostream>#include <string>#include <vector>#include "Farm.h"Farm::Farm( int spaces ){	m_Critters.reserve( spaces );}void Farm::Add( const Critter& aCritter ){	m_Critters.push_back( aCritter );}void Farm::RollCall() const{	for( std::vector< Critter >::const_iterator iter = m_Critters.begin(); iter < m_Critters.end(); ++iter )		std::cout << iter->GetName() << " here.\n";}

--------------------------------------------------------------------------------
So that's it. I think that the first way is better, but I want to be sure. Thanks in advance.

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superpig

1,826

May 11, 2008 04:54 AM

I think you want the first way, but you want to take out the unnecessary "class Farm;" from critter.h.

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sheep19

494

Author

May 11, 2008 04:57 AM

Quote:Original post by superpig
I think you want the first way, but you want to take out the unnecessary "class Farm;" from critter.h.

It's unnecessary only in this case, isn't it? If Critter used Farm in a way I would it need that?

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