Straight of the bat I understand that ANSI C is not an object orientated programming language. I want to learn how to apply a particular oo technique using c.
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For example, I want to create several audio effect classes that all have the same function names but different implementations of those functions.
If I was making this in a higher level language I would first write an interface and then implement it.
AudioEffectInterface-(float) processEffect DelayClass-(float) processEffect{ // do delay code return result}FlangerClass-(float) processEffect{ // do flanger code return result}-(void) main{ effect= new DelayEffect() effect.process() effect = new FlangerEffect() effect.process()}How can I achieve such flexibility using C?
c oop
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asked Jun 10 “11 at 9:59
dubbeatdubbeat
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There are three distinct ways you can achieve polymorphism in C:
Code it outIn the base class functions, just switch on a class type ID to call the specialized versions. An incomplete code example:
typedef enum classType { CLASS_A, CLASS_B} classType;typedef struct base { classType type;} base;typedef struct A { base super; …} A;typedef struct B { base super; …} B;void A_construct(A* me) { base_construct(&me->super); super->type = CLASS_A;}int base_foo(base* me) { switch(me->type) { case CLASS_A: return A_foo(me); case CLASS_B: return B_foo(me); default: assert(0), abort(); }}Of course, this is tedious to do for large classes.
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Store function pointers in the objectYou can avoid the switch statements by using a function pointer for each member function. Again, this is incomplete code:
typedef struct base { int (*foo)(base* me);} base;//class definitions for A and B as aboveint A_foo(base* me);void A_construct(A* me) { base_construct(&me->super); me->super.foo = A_foo;}Now, calling code may just do
base* anObject = …;(*anObject->foo)(anObject);Alternatively, you may use a preprocessor macro along the lines of:
#define base_foo(me) (*me->foo)(me)Note that this evaluates the expression me twice, so this is really a bad idea. This may be fixed, but that”s beyond the scope of this answer.
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Use a vtableSince all objects of a class share the same set of member functions, they can all use the same function pointers. This is very close to what C++ does under the hood:
typedef struct base_vtable { int (*foo)(base* me); …} base_vtable;typedef struct base { base_vtable* vtable; …} base;typedef struct A_vtable { base_vtable super; …} A_vtable;//within A.cint A_foo(base* super);static A_vtable gVtable = { .foo = A_foo, …};void A_construct(A* me) { base_construct(&me->super); me->super.vtable = &gVtable;};Again, this allows the user code to do the dispatch (with one additional indirection):
base* anObject = …;(*anObject->vtable->foo)(anObject);Which method you should use depends on the task at hand. The switch based approach is easy to whip up for two or three small classes, but is unwieldy for large classes and hierarchies. The second approach scales much better, but has a lot of space overhead due to the duplicated function pointers. The vtable approach requires quite a bit of additional structure and introduces even more indirection (which makes the code harder to read), but is certainly the way to go for complex class hierarchies.