Class design problem

h3ro · 12-18-2008, 04:29 PM

Hallo,

I am working on a program where I need to create a lot of different classes of almost the same type. But I am not sure how I should design the program so that it is easy to add a new class.

Here are the restrictions which makes it harder.
The classes has to work with the rest of the program (which is not written by me, and can not be change by me).

This is the base class which I can derive from in order to create a new class

Code:

class Man 
{
public:
	virtual void gatherInfo( /* a variable map */ ) = 0;
	virtual void doWork() = 0;
}

Right now I am doing this when I create new classes:

Code:

class IronWorker : public Man
{
	public:
		void gatherInfo( /* a variable map */ );
		void doWork();

	public:
		std::string m_name;
		int m_age;
		std::string m_workplace;

		std::string m_factoryName;
		std::string m_ironType;
}

class GarderWorker : public Man
{
	public:
		void gatherInfo( /* a variable map */ );
		void doWork();

	public:
		std::string m_name;
		int m_age;
		std::string m_workplace;

		std::string m_houseAdress;
		std::string m_floworNames[50];
		int m_kgDirtUsed;
}

When the program starts it goes into the gatherInfo() function of everyman and take the information in that map and stores it in the member variables. There are several variables that will be the same for every derived class (like name and age) but as of now I have to duplicate that code in every worker.

What I would like is to create classes more like this:

Code:

// A "new" base class
class Worker : public Man
{
public:
	void gatherInfo( /* a variable map */ );
	void doWork();

public:
	std::string m_name;
	int m_age;
	std::string m_workplace;
};

class IronWorker : public baseWorker
{
	public:
		void gatherInfo( /* a variable map */ );
		void doWork();

	public:
		std::string m_factoryName;
		std::string m_ironType;
}

class GarderWorker : public baseWorker
{
	public:
		void gatherInfo( /* a variable map */ );
		void doWork();

	public:
		std::string m_houseAdress;
		std::string m_floworNames[50];
		int m_kgDirtUsed;
}

In the code there the base variables will be gathered by the baseWorker class. But the gatherInfo function in baseWorker would never be called as it is a pure virtual function.

Any ideas on how to create a more user friendly way of doing this?
(The two virtual functions are the only two function that is called by the program)

C_ntua · 12-18-2008, 04:44 PM

I think you wanted to type this:

Code:

class baseWorker : public Man

I don't really see the problem of what you are doing.
The only "problem" seems that you won't call baseWorker gatherInfo and doWork functions ever.
So if you want to "fix" this just don't declare them. Do this:

Code:

class baseWorker : public Man
{
public:
	std::string m_name;
	int m_age;
	std::string m_workplace;
};

Note that baseWoker is also an abstract class. But that is OK with you, since you won't use it. I guess this is what you wanted.

Pure virtual functions have to be declared only once, making the class "valid" to use (not abstract anymore).

Other than that your idea is good and I don't see anything else to add

Codeplug · 12-18-2008, 04:47 PM

>> But the gatherInfo function in baseWorker would never be called...
Just because it's virtual doesn't mean you can't be explicit in what you want to call. You can "chain" the virtual calls for example:

Code:

#include <iostream>
using namespace std;

struct base
{
    virtual void print_topdown() = 0 {cout << " base" << endl;}
    virtual void print_bottomup() = 0 {cout << " base" << endl;}
};//base

struct derived : public base
{
    virtual void print_topdown() 
    {
        cout << " derived" << endl;
        base::print_topdown();
    }
    
    virtual void print_bottomup() 
    {
        base::print_bottomup();
        cout << " derived" << endl;
    }
};//derived

struct further_derived : public derived
{
    virtual void print_topdown() 
    {
        cout << " further_derived" << endl;
        derived::print_topdown();
    }

    virtual void print_bottomup() 
    {
        derived::print_bottomup();
        cout << " further_derived" << endl;
    }
};//further_derived

int main()
{
    further_derived fd;
    base *b = &fd;
    cout << "Top-Down:" << endl;
    b->print_topdown();

    cout << "\nBottom-Up" << endl;
    b->print_bottomup();
    return 0;
}//main

gg

h3ro · 12-18-2008, 04:54 PM

Codeplug: I did not know you could do that. It solved my problem perfectly! Thank you very very much

anon · 12-18-2008, 05:10 PM

That doesn't compile, at least not with GCC or Comeau.

It appears that the implementation of pure virtual functions needs to be outside of the class:

Code:

struct base
{
    virtual void print_topdown() = 0;
    virtual void print_bottomup() = 0;
};//base

void base::print_topdown()
{
    cout << " base" << endl;
}

void base::print_bottomup()
{
    cout << " base" << endl;
}

brewbuck · 12-18-2008, 05:14 PM

Quote:

Originally Posted by anon

That doesn't compile, at least not with GCC or Comeau.

It appears that the implementation of pure virtual functions needs to be outside of the class:

Why would you implement a pure virtual? That makes it not pure.

Codeplug · 12-18-2008, 05:21 PM

Provide a default implementation, and enforce derive'ers to make their own. Never used it myself, but it seemed appropriate for the example since each derivation builds on the previous.

Good catch anon - MSVC blinded me

gg

anon · 12-18-2008, 05:29 PM

Quote:

Why would you implement a pure virtual? That makes it not pure.

I think what makes it "pure" is the = 0 tagged onto the prototype. You would at least have to agree that a pure virtual destructor has to have an implementation since all base classes will have their destructor called (perhaps this is your only option to make the class abstract, since no other method is a good candidate for being pure).

Elysia · 12-19-2008, 04:52 AM

Code:

class Man 
{
public:
	virtual void gatherInfo( /* a variable map */ ) = 0;
	virtual void doWork() = 0;
}

Code:

class IronWorker : public Man
{
	public:
		virtual void gatherInfo( /* a variable map */ );
		virtual void doWork();

	public:
		std::string m_name;
		int m_age;
		std::string m_workplace;

		std::string m_factoryName;
		std::string m_ironType;
}

Another note is that while the language does not enforce you to add "virtual" to function that is a inherited virtual function, it would be good practice, since anyone looking at that class would know it is indeed virtual without looking at the base class.
Further, all your member variables are public, exposed, which makes for poor abstraction. You really need to hide them.

C_ntua · 12-19-2008, 08:26 AM

Yeah, I wanted to mention that earlier but I forgot. Since the gatherInfo(), as you say, will get information and store them and the doWork() will do the job, probably you want all the variables private. You might want another function, like printInfo() to print the variables.

h3ro · 12-19-2008, 09:10 AM

In my real program they are private. The worker classes was just something that I created as an example.

12-18-2008, 04:29 PM	#1
h3ro Registered User Join Date: Oct 2006 Location: UK/Norway Posts: 469	Class design problem Hallo, I am working on a program where I need to create a lot of different classes of almost the same type. But I am not sure how I should design the program so that it is easy to add a new class. Here are the restrictions which makes it harder. The classes has to work with the rest of the program (which is not written by me, and can not be change by me). This is the base class which I can derive from in order to create a new class Code: class Man { public: virtual void gatherInfo( /* a variable map / ) = 0; virtual void doWork() = 0; } Right now I am doing this when I create new classes: Code: class IronWorker : public Man { public: void gatherInfo( / a variable map / ); void doWork(); public: std::string m_name; int m_age; std::string m_workplace; std::string m_factoryName; std::string m_ironType; } class GarderWorker : public Man { public: void gatherInfo( / a variable map / ); void doWork(); public: std::string m_name; int m_age; std::string m_workplace; std::string m_houseAdress; std::string m_floworNames[50]; int m_kgDirtUsed; } When the program starts it goes into the gatherInfo() function of everyman and take the information in that map and stores it in the member variables. There are several variables that will be the same for every derived class (like name and age) but as of now I have to duplicate that code in every worker. What I would like is to create classes more like this: Code: // A "new" base class class Worker : public Man { public: void gatherInfo( / a variable map / ); void doWork(); public: std::string m_name; int m_age; std::string m_workplace; }; class IronWorker : public baseWorker { public: void gatherInfo( / a variable map / ); void doWork(); public: std::string m_factoryName; std::string m_ironType; } class GarderWorker : public baseWorker { public: void gatherInfo( / a variable map */ ); void doWork(); public: std::string m_houseAdress; std::string m_floworNames[50]; int m_kgDirtUsed; } In the code there the base variables will be gathered by the baseWorker class. But the gatherInfo function in baseWorker would never be called as it is a pure virtual function. Any ideas on how to create a more user friendly way of doing this? (The two virtual functions are the only two function that is called by the program)
h3ro is offline

12-18-2008, 04:44 PM	#2
C_ntua Registered User Join Date: Jun 2008 Posts: 1,285	I think you wanted to type this: Code: class baseWorker : public Man I don't really see the problem of what you are doing. The only "problem" seems that you won't call baseWorker gatherInfo and doWork functions ever. So if you want to "fix" this just don't declare them. Do this: Code: class baseWorker : public Man { public: std::string m_name; int m_age; std::string m_workplace; }; Note that baseWoker is also an abstract class. But that is OK with you, since you won't use it. I guess this is what you wanted. Pure virtual functions have to be declared only once, making the class "valid" to use (not abstract anymore). Other than that your idea is good and I don't see anything else to add
C_ntua is offline

12-18-2008, 04:54 PM	#4
h3ro Registered User Join Date: Oct 2006 Location: UK/Norway Posts: 469	Codeplug: I did not know you could do that. It solved my problem perfectly! Thank you very very much
h3ro is offline

12-18-2008, 05:21 PM	#7
Codeplug Registered User Join Date: Mar 2003 Posts: 3,903	Provide a default implementation, and enforce derive'ers to make their own. Never used it myself, but it seemed appropriate for the example since each derivation builds on the previous. Good catch anon - MSVC blinded me gg __________________ How to Ask Questions The Smart Way How to Report Bugs Effectively
Codeplug is offline

12-19-2008, 08:26 AM	#10
C_ntua Registered User Join Date: Jun 2008 Posts: 1,285	Yeah, I wanted to mention that earlier but I forgot. Since the gatherInfo(), as you say, will get information and store them and the doWork() will do the job, probably you want all the variables private. You might want another function, like printInfo() to print the variables.
C_ntua is offline

12-19-2008, 09:10 AM	#11
h3ro Registered User Join Date: Oct 2006 Location: UK/Norway Posts: 469	In my real program they are private. The worker classes was just something that I created as an example.
h3ro is offline

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