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## Help with Polymorphic Class Design

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6 replies to this topic

### #1Chris_J_H  Members

Posted 24 May 2013 - 02:39 AM

Hi - apologies if this is a bit vague but I am trying to understand the consequences of some base class design choices in my game engine. Below are 2 simplified Process classes that are designed to be used as Base Classes. It seems to me on the face of it Process1 is a better design choice as it demands way less of clients' derived classes - eg, having to remember to invoke Process1::VPause() - although I guess there is a loss of flexibility... In Derived2 for example the client could choose not to pause the process if some condition is met.

Insight/Advice appreciated - is there a better way? I really hate the routine requirement to invoke the base class VPause()... it seems to be asking for trouble.


class Process1

{

public:

virtual ~Process1();

bool GetPaused() const {return m_bPaused;}

void SetPause(bool b) {if (b!=GetPaused()) {m_bPaused = b; VPause(b);}}

protected:

virtual void VPause(bool b) { }

private:

bool m_bPaused;

};

class Derived1 : public Process1

{

//...

void VPause(bool b) override { /* do something - no need to worry about m_bPaused, invoking Process1::VPause() etc*/}

};

// --------------------------------------------------------------------------------

class Process2

{

public:

virtual ~Process2();

bool GetPaused() const {return m_bPaused;}

virtual void VPause(bool b) {m_bPaused = b;}

private:

bool m_bPaused;

};

class Derived2 : public Process2

{

//...

void VPause(bool b) override {

if (b!=GetPaused()) {

//...

}

Process2::VPause(b);}

};


### #2rip-off  Moderators

Posted 24 May 2013 - 02:54 AM

I believe this is relevant: http://www.gotw.ca/publications/mill18.htm

### #3Chris_J_H  Members

Posted 24 May 2013 - 04:04 AM

I believe this is relevant: http://www.gotw.ca/publications/mill18.htm

Good link! Gets right to the heart of the matter.... I've been taking examples for game system designs from Game Coding Complete which mostly uses Public Virtual Interface methodology and have suffered with the complexity of my derived classes. The alternative is much kinder on the smaller-brained.

### #4nfactorial  Members

Posted 24 May 2013 - 04:34 AM

Hmmm, I would say that the Process object itself shouldn't be maintaining the paused state. The process doesn't care whether or not it's paused, it just cares whether or not its update method is called.

Rather the 'paused' status of the application is managed by some higher level state of the application. For example, taking a game-engine approach, you would generally have a GameState system.

Given two game states "Active" and "Paused", the active state calls the processes update method and the paused state doesn't. This can be extended to other process management styles also ofcourse.

I just don't think 'paused' is a state that belongs to the process itself and should be externally managed by the application.

n!

Edited by nfactorial, 24 May 2013 - 04:35 AM.

### #5Chris_J_H  Members

Posted 24 May 2013 - 10:58 AM

Hmmm, I would say that the Process object itself shouldn't be maintaining the paused state. The process doesn't care whether or not it's paused, it just cares whether or not its update method is called.

Rather the 'paused' status of the application is managed by some higher level state of the application. For example, taking a game-engine approach, you would generally have a GameState system.

Given two game states "Active" and "Paused", the active state calls the processes update method and the paused state doesn't. This can be extended to other process management styles also ofcourse.

I just don't think 'paused' is a state that belongs to the process itself and should be externally managed by the application.

n!

nfactorial - yes, I think you are right if the Process doesn't need to be paused independently (which I guess mostly it doesn't....) - it can also get more complicated in some processes such as Sound Processes whereby actions must be performed entering and exiting the paused state...- however, this was just a simple example to illustrate the general concern of mixing implementation detail and interface in the same virtual functions.

### #6nfactorial  Members

Posted 24 May 2013 - 06:28 PM

Ahh, I guess I miissed what you were asking sorry. In the example I would usually call the VPause method something more like OnPause or OnPauseChanged though it's all a matter of opinion there I think the name VPause threw me as I wasn't sure what the V was for.

I generally can take either approach, depending on the situation.  I would usually base the choice on whether it is valid for the base implementation to be skipped ie. if you allow someone to overriide the function then you are stating that "if you don't want the default behaviour, don't call the base class". If the base functionality is not optional (ie. the base class implementation must be performed regardless of what a derived class wants to do) then I would do what your first example does.

n!

Edited by nfactorial, 24 May 2013 - 06:28 PM.

### #7Chris_J_H  Members

Posted 25 May 2013 - 06:52 AM

Ahh, I guess I miissed what you were asking sorry. In the example I would usually call the VPause method something more like OnPause or OnPauseChanged though it's all a matter of opinion there I think the name VPause threw me as I wasn't sure what the V was for.

I generally can take either approach, depending on the situation.  I would usually base the choice on whether it is valid for the base implementation to be skipped ie. if you allow someone to overriide the function then you are stating that "if you don't want the default behaviour, don't call the base class". If the base functionality is not optional (ie. the base class implementation must be performed regardless of what a derived class wants to do) then I would do what your first example does.

n!

Thanks - that makes sense.

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