How to write virtual function in c?

Question

In order to follow a prepared design document I want to create virtual function in C. Is there any best practice to do that? As @imreal suggested we can use function pointers to convert C structures works similar to C++ classes but how can we make sure that virtual base class functions are overrides derived class functions.

In my case I need this feature to follow the documentation but I think it can be also useful when we are converting a C++ code to C. This is required when combining a C++ code with a C code.

Create a struct with function pointers, write a functions to act as constructors to assign different functions to those pointers. Every function takes a self/this pointer to be able to access the data members. — imreal, May 25 '17 at 16:29
You mean a virtual method? C doesn't have methods; it doesn't even have classes! — ikegami, May 25 '17 at 16:36
Use an [object system](https://github.com/CObjectSystem/COS) (it's like answering "use C++" but different!). This question has no meaning without an object system - any way you find to give it meaning just means you defined your own object system instead of using an existing library or language's one. — Alex Celeste, May 25 '17 at 16:52
UseCplusPlus(); override; (this question is too abstract for C). — ThingyWotsit, May 25 '17 at 16:55

imreal · Accepted Answer · 2017-05-25T17:07:42.587

4

The language does not offer it as a feature but you can achieve the same functionality.

Create a struct with function pointers:

typedef struct Base Base_t;
struct Base {
    void (*f1)(Base_t* self);
    void (*f2)(Base_t* self);

    int dat1;
    int dat2;
};

Write a functions to act as constructors to assign different functions to those pointers.

Base constructor1()
{
    Base l = {func1, func2, 0, 0};
    return l;
}

Base constructor2()
{
    Base l = {func3, func4, 6, 13};
    return l;
}

Call the methods:

Base a = constructor1();
a.f1(&a);

Every function takes a self/this pointer to be able to access the data members.

Live example:

http://ideone.com/LPSd65

edited May 25 '17 at 17:07

answered May 25 '17 at 16:34

imreal

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It would be beneficial for readability to use better names... – Attie May 25 '17 at 16:37
At this point, why are you using C instead of C++? – Donnie May 25 '17 at 16:41
1

@Donnie There can be lots of reasons to force/prefer `C` over `C++` and simulating OOP on `C` is the topic of numerous articles, but I'm just answering the OP's question. – imreal May 25 '17 at 16:42
How can I overload base class functions in this way? – M.Khd May 25 '17 at 16:44
@M.Khd Create different functions constructors create an object with other functions. – imreal May 25 '17 at 16:45
2

@imreal - I know. We've all done it too, but it's worth asking the question so that people think about why they're choosing what they're choosing. I was perhaps a bit to flippant in the way I phrased it. – Donnie May 25 '17 at 16:49

score 2 · Answer 2 · answered May 25 '17 at 16:31

2

No you can't. 'virtual' is not part of the C vocabulary, neither is 'access level'

answered May 25 '17 at 16:31

Henri

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C++ is created by C so there should always be a way to implement C++ features in C. – M.Khd May 25 '17 at 16:45
1

@M.Khd, That makes no sense unless you are suggesting making changes to the C compiler. – ikegami May 25 '17 at 17:05
@ikegami we can add function pointers to structures to create structures that are similar to classes. So it should also be possible to modify them in a way to create virtual functions too. – M.Khd May 25 '17 at 17:22
1

@M.Khd, Never said otherwise. Of course you can write a C (or Perl or VB) program that has the same *effect* as a C++ program, and of course you can *approximate* some C++ features in those languages. But you can't actually give C those features just because C is written in C++. – ikegami May 25 '17 at 17:27
@Henri it depends on if you want to print a, so called, "kerstboom" ;) – Jan Eerland May 27 '17 at 19:10

score 1 · Answer 3 · edited Mar 23 '20 at 18:30

1

In C there is no such concept as a 'virtual' function.

Instead I would advise that you look at my answer.

Fundamentally:

Define a struct
Provide a suite of functions to act upon that struct
If a particular usage requires different functionality, then call a different function

edited Mar 23 '20 at 18:30

Ðаn

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answered May 25 '17 at 16:39

Attie

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How does this give you the dynamic behavior of virtual functions? – imreal May 25 '17 at 16:44
2

It doesn't, because that's not a thing in C. It is possible to emulate it (as in your answer), but then you end up with stuff like `self->func(self, ...)` which destroys readability. – Attie May 25 '17 at 16:47
Still, not an answer. – imreal May 25 '17 at 16:57

score 0 · Answer 4 · edited Mar 23 '20 at 18:31

There is no builtin language support, but you can get the same general functionality of using a default implementation if a special one is missing. Something like this maybe:

#include <stdio.h>

struct animal {
    const char *name;
    void (*move)(struct animal *);
};

void fly(struct animal *a)
{
    printf("soaring %s\n", a->name);
}

void walk(struct animal *a)
{
    printf("walk %s, walk\n", a->name);
}

void animal_move(struct animal *a)
{
    void (*move)(struct animal *) = a->move ? : walk;
    move(a);
}

int main(void)
{
    struct animal elephant = { .name = "elephant" };
    struct animal bird = { .name = "bird", .move = fly };

    animal_move(&elephant);
    animal_move(&bird);
    return 0;
}

How to write virtual function in c?

4 Answers4