C++ **在C中是什么意思
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what does ** mean in C
提问by numerical25
What does it mean when a object has 2 asterisks at the beginning?
对象开头有两个星号是什么意思？
**variable
采纳答案by Incognito
It is pointer to pointer. For more details you can check : Pointer to pointer
它是指向指针的指针。有关更多详细信息，您可以查看：指向指针的指针
EDITIt can be good for example for dynamically allocating multidimensional arrays :
编辑它可以很好地例如动态分配多维数组：
Like :
喜欢 ：
#include <stdlib.h>
int **array;
array = malloc(nrows * sizeof(int *));
if(array == NULL)
{
fprintf(stderr, "out of memory\n");
exit or return
}
for(i = 0; i < nrows; i++)
{
array[i] = malloc(ncolumns * sizeof(int));
if(array[i] == NULL)
{
fprintf(stderr, "out of memory\n");
exit or return
}
}
回答by Adam Rosenfield
In a declaration, it means it's a pointer to a pointer:
在声明中，这意味着它是一个指向指针的指针：
int **x; // declare x as a pointer to a pointer to an int
When using it, it deferences it twice:
使用它时，它会尊重它两次：
int x = 1;
int *y = &x; // declare y as a pointer to x
int **z = &y; // declare z as a pointer to y
**z = 2; // sets the thing pointed to (the thing pointed to by z) to 2
// i.e., sets x to 2
回答by Mavrik
Pointer to a pointer when declaring the variable.
声明变量时指向指针的指针。
Double pointer de-reference when used outside the declaration.
在声明之外使用时双指针取消引用。
回答by SLaks
It means that the variable is a pointer to a pointer.
这意味着变量是指向指针的指针。
回答by mfabish
Pointer to a pointer.
指向指针的指针。
回答by Vimal
You can use cdecl to explain C-types.
您可以使用 cdecl 来解释 C 类型。
There's an online interface here: http://cdecl.org/. Enter "int **x" into the text field and check the result.
这里有一个在线界面：http: //cdecl.org/。在文本字段中输入“int **x”并检查结果。
回答by Pavel Radzivilovsky
**variable is double dereference. If variable is an address of an address, the resulting expression will be the lvalue at the address stored in *variable.
** 变量是双重取消引用。如果变量是地址的地址，则结果表达式将是存储在 *variable 中的地址处的左值。
It can mean different things if it's a part of declaration:
如果它是声明的一部分，它可能意味着不同的事情：
type **variable would mean, on the other hand, a pointer to a pointer, that is, a variable that can hold address of another variable, which is also a pointer, but this time to a variable of type 'type'
另一方面，type **variable 意味着指向指针的指针，即可以保存另一个变量地址的变量，该变量也是一个指针，但这次指向类型为“type”的变量
回答by Achim
It means that the variable is dereferenced twice. Assume you have a pointer to a pointer to char like this:
这意味着该变量被取消引用两次。假设您有一个指向 char 的指针的指针，如下所示：
char** variable = ...;
char** 变量 = ...;
If you want to access the value this pointer is pointing to, you have to dereference it twice:
如果你想访问这个指针指向的值，你必须取消引用它两次：
**variable
**多变的
回答by Achim
It is a pointer to a pointer. You can use this if you want to point to an array, or a const char *(string). Also, in Objective-C with Cocoa this is often used to point to an NSError*.
它是一个指向指针的指针。如果您想指向一个array, 或一个const char *(字符串) ，您可以使用它。此外，在带有 Cocoa 的 Objective-C 中，这通常用于指向NSError*.
回答by HZhang
Pointer to another pointer
指向另一个指针的指针
