深入理解指针函数

1.指针函数的定义

<wbr><wbr><wbr><wbr>顾名思义，指针函数即返回指针的函数。其一般定义形式如下：</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr><wbr><wbr><strong>类型名</strong>*<strong>函数名</strong>(<strong>函数参数表列</strong>);</wbr></wbr></wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>其中，后缀运算符括号“()”表示这是一个函数，其前缀运算符星号“*”表示此函数为指针型函数，其函数值为指针，即它带回来的值的类型为指针，当调用这个函数后，将得到一个“指向返回值为…的指针（地址），“<strong>类型名</strong>”表示函数返回的指针指向的类型”。</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>“(<strong>函数参数表列</strong>)”中的括号为函数调用运算符，在调用语句中，即使函数不带参数，其参数表的一对括号也不能省略。其示例如下：</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>int*pfun(int, int);</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>由于“*”的优先级低于“()”的优先级，因而pfun首先和后面的“()”结合，也就意味着，pfun是一个函数。即：</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>int*(pfun(int, int));</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>接着再和前面的“*”结合，说明这个函数的返回值是一个指针。由于前面还有一个int，也就是说，pfun是一个返回值为整型指针的函数。</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>我们不妨来再看一看，指针函数与函数指针有什么区别？</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr>int (*pfun)(int, int);</wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>通过括号强行将pfun首先与“*”结合，也就意味着，pfun是一个指针，接着与后面的“()”结合，说明该指针指向的是一个函数，然后再与前面的int结合，也就是说，该函数的返回值是int。由此可见，pfun是一个指向返回值为int的函数的指针。</wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr>虽然它们只有一个括号的差别，但是表示的意义却截然不同。函数指针的本身是一个指针，指针指向的是一个函数。指针函数的本身是一个函数，其函数的返回值是一个指针。</wbr></wbr></wbr></wbr>

2.<wbr><wbr><wbr></wbr></wbr></wbr>用函数指针作为函数的返回值

<wbr><wbr><wbr></wbr></wbr></wbr>在上面提到的指针函数里面，有这样一类函数，它们也返回指针型数据（地址），但是这个指针不是指向int、char之类的基本类型，而是指向函数。对于初学者，别说写出这样的函数声明，就是看到这样的写法也是一头雾水。比如,下面的语句：

<wbr><wbr><wbr><wbr>int(*ff(int))(int *, int);</wbr></wbr></wbr></wbr>

我们用上面介绍的方法分析一下，ff首先与后面的“()”结合，即：

<wbr><wbr><wbr><wbr>int(*<u>(ff(int))</u>)(int *, int);<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>//用括号将ff(int)再括起来</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

也就意味着，ff是一个函数。

<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>接着与前面的“*”结合，说明ff函数的返回值是一个指针。然后再与后面的“()”结合，也就是说，该指针指向的是一个函数。

这种写法确实让人非常难懂，以至于一些初学者产生误解，认为写出别人看不懂的代码才能显示自己水平高。而事实上恰好相反，能否写出通俗易懂的代码是衡量程序员是否优秀的标准。一般来说，用typedef关键字会使该声明更简单易懂。在前面我们已经见过：

<wbr><wbr><wbr><wbr>int(*PF)(int *, int);</wbr></wbr></wbr></wbr>

也就是说，PF是一个函数指针“变量”。当使用typedef声明后，则PF就成为了一个函数指针“类型”，即：

<wbr><wbr><wbr><wbr>typedefint (*PF)(int *, int);</wbr></wbr></wbr></wbr>

这样就定义了返回值的类型。然后，再用PF作为返回值来声明函数:

<wbr><wbr><wbr>PF ff(int);</wbr></wbr></wbr>

<wbr><wbr><wbr></wbr></wbr></wbr>下面将以程序清单1为例，说明用函数指针作为函数的返回值的用法。当程序接收用户输入时，如果用户输入d，则求数组的最大值，如果输入x，则求数组的最小值，如果输入p，则求数组的平均值。

程序清单1<wbr></wbr> 求最值与平均值示例

1<wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>#include<stdio.h>

2<wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>#include <assert.h>

3<wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>double GetMin(double *dbData, int iSize)<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>// <span style="color:black">求最小值</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

4<wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>{

5<wbr><wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>doubledbMin;</wbr></wbr></wbr></wbr>

6<wbr><wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>inti;</wbr></wbr></wbr></wbr>

7<wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>

8<wbr><wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>assert(iSize&gt;0);</wbr></wbr></wbr></wbr>

9<wbr><wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>dbMin=dbData[0];</wbr></wbr></wbr></wbr>

10<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>for(i=1; i&lt;iSize; i++){</wbr></wbr></wbr></wbr></wbr>

11<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>if (dbMin&gt;dbData[i]) {</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

12<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>dbMin=dbData[i];</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

13<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>}</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

14<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>}</wbr></wbr></wbr></wbr></wbr>

15<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>returndbMin;</wbr></wbr></wbr></wbr></wbr>

16<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>}

17

18<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>double GetMax(double *dbData, int iSize)<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>// <span style="color:black">求最大值</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

19<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>{

20<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>doubledbMax;</wbr></wbr></wbr></wbr>

21<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>inti;</wbr></wbr></wbr></wbr>

22

23<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>assert(iSize&gt;0);</wbr></wbr></wbr></wbr>

24<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>dbMax=dbData[0];</wbr></wbr></wbr></wbr>

25<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>for(i=1; i&lt;iSize; i++){</wbr></wbr></wbr></wbr>

26<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>if (dbMax&lt; dbData[i]) {</wbr></wbr></wbr></wbr></wbr></wbr></wbr>

27<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>dbMax=dbData[i];</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

28<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr>}</wbr></wbr></wbr>

29<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>}</wbr></wbr></wbr></wbr>

30<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>returndbMax;</wbr></wbr></wbr></wbr>

31<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>}

32

33<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>double GetAverage(double *dbData, int iSize)<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>//<span style="color:black">求平均值</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

34<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>{

35<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>doubledbSum=0;</wbr></wbr></wbr></wbr>

36<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>inti;</wbr></wbr></wbr></wbr>

37<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>

38<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>assert(iSize&gt;0);</wbr></wbr></wbr></wbr>

39<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>for(i=0; i&lt;iSize; i++)</wbr></wbr></wbr></wbr>

40<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>{</wbr></wbr></wbr></wbr>

41<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>dbSum+=dbData[i];</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

42<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>}</wbr></wbr></wbr></wbr>

43<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>returndbSum/iSize;</wbr></wbr></wbr></wbr>

44<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>}

45

46<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>double UnKnown(double *dbData, int iSize)<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>//<span style="color:black">未知算法</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

47<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>{

48<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>return0;</wbr></wbr></wbr></wbr>

49<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>}

50

51<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>typedef double (*PF)(double *dbData, int iSize);<wbr><wbr><wbr><wbr>// <span style="color:black">定义函数指针类型</span></wbr></wbr></wbr></wbr>

52<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>PF GetOperation(char c)<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>// <span style="color:black">根据字符得到操作类型，返回函数指针</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

53<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>{

54<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>switch(c)</wbr></wbr></wbr></wbr>

55<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr>{</wbr></wbr></wbr></wbr>

56<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>case 'd':</wbr></wbr></wbr></wbr></wbr>

57<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>return GetMax;</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

58<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr>case'x':</wbr></wbr></wbr></wbr></wbr></wbr>

59<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>return GetMin;</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

60<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr>case'p':</wbr></wbr></wbr></wbr></wbr></wbr>

61<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>return GetAverage;</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

62<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr>default:</wbr></wbr></wbr></wbr></wbr></wbr>

63<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>return UnKnown;</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

64<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr><wbr>}</wbr></wbr></wbr></wbr></wbr></wbr>

65<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>}

66

67<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>int main(void)

68<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>{

69<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>doubledbData[]={3.1415926, 1.4142, -0.5,999, -313, 365};</wbr></wbr></wbr></wbr></wbr>

70<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>intiSize=sizeof(dbData)/sizeof(dbData[0]);</wbr></wbr></wbr></wbr></wbr>

71<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>charc;</wbr></wbr></wbr></wbr></wbr>

72

73<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>printf("Pleaseinput the Operation :\n");</wbr></wbr></wbr></wbr></wbr>

74<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr>c=getchar();

75<wbr><wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr><wbr><wbr><wbr>printf("resultis %lf\n", GetOperation(c)(dbData,iSize));<wbr><wbr>// <span style="color:black">通过函数指针调用函数</span></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

76<wbr><wbr><wbr><wbr></wbr></wbr></wbr></wbr>}

<wbr><wbr><wbr><wbr>上述程序中前面</wbr></wbr></wbr></wbr>4个函数分别实现求最大值、最小值、平均值和未知算法，然后实现了GetOperation函数。这个函数根据字符的返回值实现上面4个函数。它是以函数指针的形式返回的，从后面的main函数的GetOperation(c)(dbData, iSize)可以看出，通过这个指针可以调用函数。