types int **和int [] []之间有什么区别？ [英] What is difference between types int** and int[][]?
问题描述
如果以下分配有效：
int a[2] = {1,2};
int* b = a;
那么这有什么问题：
int a[2][2]={1,2,3,4};
int** b = a;
C ++给出一个错误，它不能转换 int [] [] 到 int ** 。如果 int [] 与 int * ？
C++ gives an error that it can't convert int[][] to int**. What is difference between the two types if int[] is the same as int*?
推荐答案
简单。它只是一个编译器错误。数组很棘手。以下是规则：
Take it easy. It is only a compiler error. Arrays are pretty tricky. Here is the rule:
到此数组的元素地址
您的第一个代码段如下：
Your first snippet looks like:
int a[2] = {1,2};
因此，如果 a 在赋值的右边，它衰减到元素零的地址，这就是为什么它有类型 int * 。这会带给您
So according to the rule if a is in the right hand side of a assignment then it decays to address of the element zero and that is why it has type int *. This brings you to
int *b = a;
在第二个片段中，你真正拥有的是一个数组数组。 （顺便说一句，为了使它明确我已经改变了你的代码。）
In the second snippet what you really have is an array of arrays. (By the way, to make it explicit I've changed your code a bit.)
int a[2][2]={{1,2},{3,4}};
此时 a 到两个整数的数组！因此，如果你想为某些东西分配 a ，你需要这个东西具有相同的类型。
This time a will decay to the pointer to an array of two integers! So if you would want to assign a to something, you would need this something to have the same type.
int (*b)[2] = a; //Huh!
（这种语法可能有点令人惊讶，但只是想想一会儿， code> int * b [2]; 得到点？ b 将是一个指向整数的数组！想要...）
(This syntax maybe a bit stunning to you, but just think for a moment that we have written int *b[2]; Got the point? b would be an array of pointers to integers! Not really what we wanted...)
你可以在这里停止阅读，但你也可以继续，因为我没有告诉你所有的真相。我提到的规则有三个例外...
You could stop reading here, but you could also move on, because I have not told you all the truth. The rule I mentioned has three exceptions...
数组的值将不衰减到地址
- 数组是
sizeof - array是
&
的操作数数组是一个字符串初始值字符数组
让我们更详细地解释这些异常和例子：
Let's explain these exceptions in more detail and with examples:
int a[2];
int *pi = a ; /* the same as pi = &a[0]; */
printf("%d\n", sizeof(a)); /* size of the array, not of a pointer is printed! */
int (*pi2)[2] = &a; /* address of the array itself is taken (not the address of a pointer) */
char a[] = "Hello world ";
这里不是指向Hello world的指针，而是复制整个字符串， 。
Here not a pointer to "Hello world" is copied, but the whole string is copied and a points to this copy.
有很多信息，而且很难一次性理解所有的内容，所以请抽出时间。我建议您阅读本主题的K& R，之后这本精彩的书。
There is really a lot of information and it is really difficult to understand everything at once, so take your time. I recommend you to read K&R on this topic and afterwards this excellent book.
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