C 的直接多态性

Question

Is it possible to make polymorphic functions with C where we don't have to use any classes or OOP type function calls in the main function?

就像，我想做一个 function 来返回两个整数或字符或字符串中的最小值。 我只想在主 function 中调用 function 之类的function_name(variable1, variable2) 。

使用 C++ 模板很容易做到这一点，但有没有办法用 C 做到这一点？ 我已经仔细阅读了所有与“C 多态性”相关的问题和答案，但找不到任何线索。

Answer 1

有时#define可以做你想做的事。

但它使用起来既困难又复杂。 以 linux 中的 max 宏为例：

从 linux 5.12-rc2： /include/linux/minmax.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_MINMAX_H
#define _LINUX_MINMAX_H

/*
 * min()/max()/clamp() macros must accomplish three things:
 *
 * - avoid multiple evaluations of the arguments (so side-effects like
 *   "x++" happen only once) when non-constant.
 * - perform strict type-checking (to generate warnings instead of
 *   nasty runtime surprises). See the "unnecessary" pointer comparison
 *   in __typecheck().
 * - retain result as a constant expressions when called with only
 *   constant expressions (to avoid tripping VLA warnings in stack
 *   allocation usage).
 */
#define __typecheck(x, y) \
    (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))

/*
 * This returns a constant expression while determining if an argument is
 * a constant expression, most importantly without evaluating the argument.
 * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
 */
#define __is_constexpr(x) \
    (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

#define __no_side_effects(x, y) \
        (__is_constexpr(x) && __is_constexpr(y))

#define __safe_cmp(x, y) \
        (__typecheck(x, y) && __no_side_effects(x, y))

#define __cmp(x, y, op) ((x) op (y) ? (x) : (y))

#define __cmp_once(x, y, unique_x, unique_y, op) ({ \
        typeof(x) unique_x = (x);       \
        typeof(y) unique_y = (y);       \
        __cmp(unique_x, unique_y, op); })

#define __careful_cmp(x, y, op) \
    __builtin_choose_expr(__safe_cmp(x, y), \
        __cmp(x, y, op), \
        __cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))

/**
 * min - return minimum of two values of the same or compatible types
 * @x: first value
 * @y: second value
 */
#define min(x, y)   __careful_cmp(x, y, <)

/**
 * max - return maximum of two values of the same or compatible types
 * @x: first value
 * @y: second value
 */
#define max(x, y)   __careful_cmp(x, y, >)

/**
 * min3 - return minimum of three values
 * @x: first value
 * @y: second value
 * @z: third value
 */
#define min3(x, y, z) min((typeof(x))min(x, y), z)

/**
 * max3 - return maximum of three values
 * @x: first value
 * @y: second value
 * @z: third value
 */
#define max3(x, y, z) max((typeof(x))max(x, y), z)

C 语言本身没有适当的方法来处理这个问题。

对于轻量级项目，您可以简单地为每种类型实现这样的 function：

int min(int a, int b) {...}
short min(short a, short b) {...}
float min(float a, float b) {...}
...