二维/多维数组的内存映射 [英] MEMORY MAP for 2d / multidimensional arrays

问题描述

任何人都可以直观地解释一下二维数组如何存储在内存中:a , &a, &a[0] , a[0] 都具有相同的地址...这似乎是一个以某种方式指向自己的指针……这不可能是对的……这已经困扰我近一年了，在网上搜索也让我找不到合适的答案.....帮助真的很感激....thanx

Can anyone please explain visually as to how the 2D array is stored in memory as: a , &a, &a[0] , a[0] all have the same address... It seems like a pointer pointing to itself in a way...and that cant be right... This has been bugging me for nearly a year and searching on the web has lead me to no apt answer as well..... help is really appreciated....thanx

enter code here

#include<stdio.h>

int main()
{
    int a[2][3]={10,20,30,40,50,60};

    int row =0,col=0;

    printf("&a = %d ",&a);    
    printf("
a = %d ",a);
    printf("
&a[0] = %d ",&a[0]);
    printf("
a[0] = %d ",a[0]);
    printf("
&a[1] = %d ",&a[1]);
    printf("
a[1] = %d ",a[1]);
    printf("
&a[0][0] = %d ",&a[0][0]);

    int *p;
    printf("

 sizeof(p) = %d ",sizeof(p) );

    printf("

 sizeof(a) = %d ",sizeof(a) );
    printf("

 sizeof(&a) = %d ",sizeof(&a) );
    printf("

 sizeof(&a[0]) = %d ",sizeof(&a[0]) );
    printf("

 sizeof(a[0]) = %d ",sizeof(a[0]) );
    printf("

 sizeof(&a[1]) = %d ",sizeof(&a[1]) );
    printf("

 sizeof(a[1]) = %d ",sizeof(a[1]) );
    printf("

 sizeof(&a[0][0]) = %d ",sizeof(&a[0][0]) );
}

<小时>

输出

&a = 2293536
a = 2293536
&a[0] = 2293536
a[0] = 2293536
&a[1] = 2293548
a[1] = 2293548
&a[0][0] = 2293536

sizeof(p) = 4

sizeof(a) = 24

sizeof(&a) = 4

sizeof(&a[0]) = 4

sizeof(a[0]) = 12

sizeof(&a[1]) = 4

sizeof(a[1]) = 12

sizeof(&a[0][0]) = 4

不要让我参考二维数组的内存映射C ...那没有帮助...

Don't refer me to Memory map for a 2D array in C ... that didnt help ...

推荐答案

所以数组 a 是一个对象占用一块内存:

So the array a is one object taking up a block of memory:

| a                                                         |

这是一个长度为 2 的数组，所以如果我们绘制组成它的元素，它看起来像这样:

It's an array of length 2, so if we draw in the elements that make it up it looks like this:

| a[0]                        | a[1]                        |

a[0] 依次是一个长度为 3 的数组，如下所示:

a[0] in turn is an array of length 3, and looks like this:

| a[0][0] | a[0][1] | a[0][2] |

a[1] 看起来是一样的，所以我们可以重绘数组 a 看起来像这样:

a[1] looks the same, so we can redraw the array a to look like this:

| a[0][0] | a[0][1] | a[0][2] | a[1][0] | a[1][1] | a[1][2] |

注意 a、a[0] 和 a[0][0] 都位于内存中的同一点:对象 a 的开始.但是，它们确实有不同的大小:a 是整个二维数组"，a[0] 是常规数组，而 a[0][0] 是单个 `int.

Notice that a, a[0] and a[0][0] are all located at the same point in memory: the start of the object a. However, they do have different sizes: a is the entire "2D array", a[0] is a regular array, and a[0][0] is a single `int.

这就解释了为什么 &a、&a[0] 和 &a[0][0] 是相同的地址(尽管它们有不同的类型)——它们是位于内存中同一点的事物的地址.

This explains why &a, &a[0] and &a[0][0] are the same addresses (though they have different types) - they're the addresses of things located at the same point in memory.

此外，有一条规则是，如果数组在不是一元 & 或 sizeof 运算符的主题的表达式中计算，则计算为指针到它的第一个元素:也就是说，使用普通的 a 等效于 &a[0].由于 a[0] 也是一个数组，使用普通的 a[0] 相当于 &a[0][0].这解释了为什么 a 和 a[0]也评估为与 &a、&a[0] 和 &a[0][0] 相同的地址.

Additionally, there's a rule that if an array is evaluated in an expression where it's not the subject of the unary & or sizeof operators, it evaluates to a pointer to its first element: that is, using a plain a is equivalent to &a[0]. Since a[0] is also an array, using a plain a[0] is equivalent to &a[0][0]. This explains why a and a[0] also evaluate to the same addresses as &a, &a[0] and &a[0][0].

数组的地址和数组中第一个元素的地址相同这一事实并不令人惊讶:同样的事情也发生在 struct 上.鉴于:

The fact that the address of an array and the adddress of the first element in that array are the same isn't really surprising: the same thing happens with struct, too. Given:

struct { int a; int b; } x;

您会发现 &x 和 &x.a 是相同的地址(尽管类型不同).

You'll find that &x and &x.a are the same address (albeit with different types).

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