# 函数指针

## C/C++编程语言

C语言标准规定，函数指示符（function designator，即函数名字）既不是左值，也不是右值。但C++语言标准规定函数指示符属于左值，因此函数指示符转换为函数指针的右值属于左值转换为右值。

### 实例

```/* 例一：函式指標直接呼叫 */

# ifndef __cplusplus
# include <stdio.h>
# else
# include <cstdio>
# endif

int max(int x, int y)
{
return x > y ? x : y;
}

int main(void)
{
/* p 是函式指標 */
int (* p)(int, int) = & max; // &可以省略
int a, b, c, d;

scanf("%d %d %d", & a, & b, & c);

/* 與直接呼叫函式等價，d = max(max(a, b), c) */
d = p(p(a, b), c);

printf("the maxumum number is: %d\n", d);

return 0;
}
```

```/* 例二：函式指標作為參數 */

struct object
{
int data;
};

int object_compare(struct object * a,struct object * z)
{
return a->data < z->data ? 1 : 0;
}

struct object *maximum(struct object * begin,struct object * end,int (* compare)(struct object *, struct object *))
{
struct object * result = begin;
while(begin != end)
{
if(compare(result, begin))
{
result = begin;
}
++ begin;
}
return result;
}

int main(void)
{
struct object data[8] = ((0}, {1}, {2}, {3}, {4}, {5}, {6}, {7));
struct object * max;
max = maximum(data + 0, data + 8, & object_compare);
return 0;
}
```

## 脚注

1. ^ C++语言标准规定：A function designator is an expression that has function type. Except when it is the operand of the sizeof operator or the unary & operator, a function designator with type ‘‘function returning type’’ is converted to an expression that has type ‘‘pointer to function returning type’’.