二维数组和二维指针


原文地址:http://www.cppblog.com/glxhyt/archive/2011/11/21/160668.html

入职一年了,这一年自己学到许多,但是忘记也很多,于是决定定下心来整理以前学到的,并且继续学习

    
    二维数组和二级指针,这真是头疼的问题,困扰了我好几次,
   
    先转一下 wanpengcoder 的二维数组和二级指针 

前两天写个程序,传参数的时候想传个二维数组进去,结果悲剧了,函数写成Fun (int **p){},原来没有这么写过,

以为这么写也是对的,结果错了,查了些资料,做个总结。

Fun (int **p){}这里面的int **p //这里的p不是二维数组的指针,而是指向指针的指针,即二级指针。

正确的二维数组的指针应该是:Int a[2][2];Int (*p)[2];//定义时无论数组维数,只可忽略第一维

例如:int a[2][2]={0,1,2,3};

int **p=(int**)a;//强制将二维数组指针转为指向指针的指针

则此时p[0]=0;p[1]=1;p[2]=2;p[3]=3;

而p[0][0]=*(*(p+0)+0)=**p;

p[0][1]=*(*(p+0)+1);

对于p[0][0]:由于*p=0; ====> **p=*(0);引用地址为零的内存,必然是错误的。

对于p[0][1]=*(*p+1)====>*(4),引用了非法内存同样,

对于p[1][0]=*(1),p[1][1]=*(5),均引用了非法内存所以说,二位数组并不能简单的转换成指向指针的指针。

二维数组其实只是一个指针,而二级指针是指向指针的指针,所以二者并不等价。如上例所示:int a[2][2];

a是指向整个数组的首地址,并不是int **;所以不要指望向函数fun里面传实参 p=a;

 

感謝sld666666,我覺得那個應該是和下面的情況類似把,中間有個強制轉換的過程:

 

#include <iostream>

 

void fun(char ** p)

{

char (*p1)[10] = (char(*)[10])p;

std::cout<<p1[0][0]<<std::endl;

}

 

int main(int argc, char* argv[])

{

char data[][10] = {"abc","def"};

fun((char **)data);

return 0;

}

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<c程序设计语言>中的关于这个的解释:

Newcomers to C are sometimes confused about the difference between a two-dimensional array and an array of pointers, such as name in the example above. Given the definitions

int a[10][20];

int *b[10];

then a[3][4] and b[3][4] are both syntactically legal references to a single int. But a is a true two-dimensional array: 200 int-sized locations have been set aside, and the conventional rectangular subscript calculation 20 * row +col is used to find the element a[row,col]. For b, however, the definition only allocates 10 pointers and does not initialize them; initialization must be done explicitly, either statically or with code. Assuming that each element of b does point to a twenty-element array, then there will be 200 ints set aside, plus ten cells for the pointers. The important advantage of the pointer array is that the rows of the array may be of different lengths. That is, each element of b need not point to a twenty-element vector; some may point to two elements, some to fifty, and some to none at all.

Although we have phrased this discussion in terms of integers, by far the most frequent use of arrays of pointers is to store character strings of diverse lengths, as in the function month_name. Compare the declaration and picture for an array of pointers:

char *name[] = { "Illegal month", "Jan", "Feb", "Mar" };


with those for a two-dimensional array:

char aname[][15] = { "Illegal month", "Jan", "Feb", "Mar" };




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