How could one implement C++ virtual functions in C

Question

The C++ language provides virtual functions. Within the constraints of a pure C language implementation, how can a similar effect be achieved?

Answer 1

Stolen from here.

From the C++ class

class A {
protected:
    int a;
public:
    A() {a = 10;}
    virtual void update() {a++;}
    int access() {update(); return a;}
};

a C code fragment can be derived. The three C++ member functions of class A are rewritten using out-of-line (standalone) code and collected by address into a struct named A_functable. The data members of A and combined with the function table into a C struct named A.

struct A;

typedef struct {
    void (*A)(struct A*);
    void (*update)(struct A*);
    int (*access)(struct A*);
} A_functable;

typedef struct A{
    int a;
    A_functable *vmt;
} A;

void A_A(A *this);
void A_update(A* this);
int A_access(A* this);

A_functable A_vmt = {A_A, A_update, A_access};

void A_A(A *this) {this->vmt = &A_vmt; this->a = 10;}
void A_update(A* this) {this->a++;}
int A_access(A* this) {this->vmt->update(this); return this->a;}

/*
class B: public A {
public:
    void update() {a--;}
};
*/

struct B;

typedef struct {
    void (*B)(struct B*);
    void (*update)(struct B*);
    int (*access)(struct A*);
} B_functable;

typedef struct B {
    A inherited;
} B;

void B_B(B *this);
void B_update(B* this);

B_functable B_vmt = {B_B, B_update, A_access};

void B_B(B *this) {A_A(this); this->inherited.vmt = &B_vmt; }
void B_update(B* this) {this->inherited.a--;}
int B_access(B* this) {this->inherited.vmt->update(this); return this->inherited.a;}

int main() {
    A x;
    B y;
    A_A(&x);
    B_B(&y);
    printf("%d\n", x.vmt->access(&x));
    printf("%d\n", y.inherited.vmt->access(&y));
}

More elaborate than necessary, but it gets the point across.

Answer 2

@GCC....A virtual function is declared in the Base class of an object and is then "overriden" or implemented in the sub classes. i.e., say you have Vehicle Base class and you create two sub-classes, Motorcycle and, Automobile. The Base class would declare a virtual function of AddTires() Then the Sub Classes would implement this function and each sub class would implement it differently. A car has 4 wheels, where a motorcycle has 2. I can't give you the syntax for C or C++, though. Hope this helps

Answer 3

Here is a description of what virtual functions are.

There is no way to implement virtual functions in plain C, because C has no notion of inheritance.

Update: As is discussed in the comments below, it is possible to do something similar to virtual functions in straight C using structures and function pointers. However, if you are accustomed to a language like C++ that has "true" virtual functions, you will probably find the C approximation far less elegant and harder to use.

Answer 4

Virtual functions are a feature of C++'s object orientation. They refer to methods that depend on a specific object instance rather than what type you're currently carrying them around as.

In other words: if you instantiate an object as Bar, then cast it to Foo, virtual methods will still be the ones they were at instantiation (defined in Bar), while other methods will be the ones from Foo.

Virtual functions are typically implemented by way of vtables (that's for you to do more research on ;)).

You can simulate similar things in C by using structs as poor man's objects and saving function pointers in them.

(More correctly, non-virtual functions make it ambiguous which class the method should be taken from, but in practice I believe C++ uses the current type.)