多态性(在 C 中) [英] Polymorphism (in C)

问题描述

可能重复:
如何在 C 中模拟 OO 风格的多态性?

我试图通过我所知道的语言中的示例来更好地理解多态性的概念；C语言中是否存在多态性?

I'm trying to better understand the idea of polymorphism with examples from languages I know; is there polymorphism in C?

推荐答案

这是 Nekuromento 的第二个例子，考虑到我认为面向对象 C 的惯用方式:

This is Nekuromento's second example, factored in the way I consider idiomatic for object-oriented C:

#ifndef ANIMAL_H_
#define ANIMAL_H_

struct animal
{
    // make vtable_ a pointer so they can be shared between instances
    // use _ to mark private members
    const struct animal_vtable_ *vtable_;
    const char *name;
};

struct animal_vtable_
{
    const char *(*sound)(void);
};

// wrapper function
static inline const char *animal_sound(struct animal *animal)
{
    return animal->vtable_->sound();
}

// make the vtables arrays so they can be used as pointers
extern const struct animal_vtable_ CAT[], DOG[];

#endif

cat.c

#include "animal.h"

static const char *sound(void)
{
    return "meow!";
}

const struct animal_vtable_ CAT[] = { { sound } };

dog.c

#include "animal.h"

static const char *sound(void)
{
    return "arf!";
}

const struct animal_vtable_ DOG[] = { { sound } };

main.c

#include "animal.h"
#include <stdio.h>

int main(void)
{
    struct animal kitty = { CAT, "Kitty" };
    struct animal lassie = { DOG, "Lassie" };

    printf("%s says %s
", kitty.name, animal_sound(&kitty));
    printf("%s says %s
", lassie.name, animal_sound(&lassie));

    return 0;
}

这是一个运行时多态性的例子，因为那是方法解析发生的时候.

This is an example of runtime polymorphism as that's when method resolution happens.

C1x 添加了泛型选择，这使得通过宏实现编译时多态性成为可能.以下示例摘自 C1x 4 月草案，第 6.5.1.1 节第 5 节:

C1x added generic selections, which make compile-time polymorphism via macros possible. The following example is taken from the C1x April draft, section 6.5.1.1 §5:

#define cbrt(X) _Generic((X), 
    long double: cbrtl, 
    default: cbrt, 
    float: cbrtf 
)(X)

用于数学函数的类型通用宏已经在 C99 中通过头文件 tgmath.h 提供，但用户无法在不使用编译器扩展的情况下定义自己的宏.

Type-generic macros for math functions were already available in C99 via the header tgmath.h, but there was no way for users to define their own macros without using compiler extensions.

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