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# Modify functions from generic class? (c++)

## 6 posts in this topic

Hi

I have many classes in my gameengine. They perform verious standard things (used in all games) but i also want to be able to do game-specific stuff with them, preferably without making inherited classes from the generic class.

Can i declare an "empty function" for the class that i fill differently depending on the game?

And maybe also add some game-specific variables?

Thanks

Erik

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What language?

Why are you trying to avoid inheritance? You could do something like this using function pointers but it would be about as pointless as possible.

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c++ is the language (see topic)

Since i want to remain uniformity. And be able to reuse some stuff. Often i want to do very similar stuff just modify something.

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Since the question is extremely generic the answers will end up being extremely generic:
- use composition
- use polymorphism despite your reservations
- use compile-time polymorphism
- embed a scripting language
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Maybe consider using entity-component system for your game engine (i.e. composition, mentioned by BitMaster).

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Assuming you are writing good code and predominantly using interface inheritance instead of implementation inheritance, you can always derive from an interface that provides your "general" functionality, and have each specific game add an interface for its own specific functionality.

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Can i declare an "empty function" for the class that i fill differently depending on the game?

At compile time? Sure - just declare the function in the engine's header files, but only define it in the game-specific source files.
//MyClass.h - Engine header file
class MyClass
{
public:
void function();
void gameSpecificFunction();
};
.
//MyClass.cpp - Engine source file
void MyClass::function()
{
//Code....
}
.
//MyClass_Game.cpp - Game-specific source file.
void MyClass::gameSpecificFunction()
{
//Code....
}
.
If you mean at runtime you want the function to change? Sure - just use function pointers.
typedef std::function<void(float,int)> MyFunction;

class MyClass
{
public:
void SetFunctionToUse(MyFunction function) { functionToCall = function; }

void CallFunction(float myFloat, int myInt)
{
//Check if a function has been set.
if(functionToCall)
{
//If so, call the function.
functionToCall(myFloat, myInt);
}
}

private:
MyFunction functionToCall;
};
.

And maybe also add some game-specific variables?

Well, that's slightly more difficult, but yes. You can use the pImpl idiom very effectively for this.

//MyClass.h - Engine header file
class MyClass
{
public:
void function();
void gameSpecificFunction();

private:
int normalVariable;
//The game-specific variables don't go here yet.

private:
//We tell the compiler a class called 'MyClass::Implementation' exists, but we don't define it yet.
//This is a class within a class (in terms of namespace).
class Implementation;

std::unique_ptr<Implementation> pImpl; //A pointer to the 'Implementation' class that we haven't yet defined.
};
.
//MyClass.cpp - Engine source file

//I'm including a _source_ file here, not a header file.
#include "../Game/MyClass_Implementation.cpp"

MyClass::gameSpecificFunction()
{
//Call the Implementation's version of the function.
pImpl->realFunction();
}

.
//MyClass_Implementation.cpp - Game-specific source file

//NOW we define MyClass::Implementation.
//I like to make pImpl's be 'structs' since they are public by default.
struct MyClass::Implementation
{
//Game-specific variables.
int meow = 357;

//Game-specific function implementation
void realFunction()
{
//...code...
}

};
.
However, using these in some situations may indicate bad architecture - I would give it some serious thought and ask myself what I am actually needing, why I'm needing it, and what would be the best way to implement it. Edited by Servant of the Lord
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