C++中的**是什么？
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What is ** in C++?
提问by agscala
I've seen some code, as well as some errors generated from my compiler that have a '**' token before the variable (eg **variablename unreferenced-- or something, I can't recall exactly offhand). I'm fairly certain this is related to pointers, if I had to guess it looks like it's trying to dereference twice. '**' is fairly ungoogleable. Can someone point me to a good website/documentation or would someone care to explain it here?
我看过一些代码，以及我的编译器生成的一些错误，这些错误**在变量之前有一个 ' ' 标记（例如 **variablename unreferenced-- 或者其他东西，我无法准确地回忆起来）。我相当肯定这与指针有关，如果我不得不猜测它看起来像是试图取消引用两次。' **' 是相当不可搜索的。有人可以给我指出一个好的网站/文档，还是有人愿意在这里解释一下？
Thanks.
谢谢。
Great responses. If I can add, what would be some situations where it is useful to have a pointer to a pointer? Shouldn't you just be using the original pointer instead of creating yet another pointer to the original pointer?
很棒的反应。如果我可以添加，在哪些情况下使用指向指针的指针是有用的？您不应该只使用原始指针而不是创建另一个指向原始指针的指针吗？
回答by Antti Huima
** is not actually only pointer to pointer (as in declaration), but is also the dereference of a dereference (in a statement).
** 实际上不仅是指向指针的指针（如在声明中），而且还是对解引用的解引用（在语句中）。
It is used often in C which does not have the & notation for references, e.g. to update a return value which is a pointer type:
它经常在 C 中使用，它没有引用的 & 符号，例如更新一个指针类型的返回值：
int alloc_foo(struct foo **foo_ret)
{
*foo_ret = malloc(sizeof(struct foo));
return 1; /* to indicate success; return value in foo_ret */
}
回答by Parappa
You may recognize the signature for main():
您可能会认出 main() 的签名：
int main(int argc, char* argv[])
The following is equivalent:
以下是等效的：
int main(int argc, char** argv)
In this case, argv is a pointer to an array of char*.
在这种情况下，argv 是一个指向 char* 数组的指针。
In C, the index operator [] is just another way of performing pointer arithmetic. For example,
在 C 中，索引运算符 [] 只是执行指针运算的另一种方式。例如，
foo[i]
produces the same code as
产生相同的代码
*(foo + i)
回答by Rob Kennedy
It's not a **token. It's simply a *token followed by another *token. In your case, you have a pointer to a pointer, and it's being dereferenced twice to get whatever's really being pointed to.
它不是**令牌。它只是一个*令牌，然后是另一个*令牌。在您的情况下，您有一个指向指针的指针，并且它被取消引用两次以获取真正指向的内容。
回答by Can Berk Güder
**is a pointer to a pointer.
**是一个指向指针的指针。
It might be a matrix (an array of arrays) or an array of strings (a chararray), etc.
它可能是一个矩阵（数组数组）或字符串char数组（数组）等。
回答by Paul Beckingham
It's a double dereference.
这是一个双重取消引用。
int i = 3;
int* ptr_to_i = &i;
int** ptr_to_ptr_to_i = &ptr_to_i;
std::cout << **ptr_to_ptr_to_i << std::endl;
Prints 3.
打印 3。
回答by P Daddy
I just wanted to underscore some of the uses for a pointer to a pointer. Most of these aretouched on by other posts, but I thought reiteration might help.
我只是想强调指向指针的一些用途。其中大部分内容都在其他帖子中有所涉及，但我认为重申可能会有所帮助。
It allows a callee to modifya pointer owned by the caller. For example, one could pass a pointer to a pointer to the beginning of a string, and the callee could modify the pointed-to pointer to now point to a position within the string where a particular character occurs.
Because arrays degrade to pointers (and pointers can be treated as arrays), you will often see a pointer to a pointer if you have:
A pointer to an array. This is a generalization of the above case, since a "string" (a C-style string, anyway) is really just an array of
chars.An array of pointers. You might, for example, have an array of pointers to objects, allowing for polymorphism, or an array of pointers to select objects stored in another collection.
An array of arrays. Again, arrays degrade to pointers, so this is a specific case of the above. This is often used for so called "jagged" arrays (as opposed to rectangular).
它允许被调用者修改调用者拥有的指针。例如，可以传递一个指向字符串开头的指针的指针，被调用者可以修改所指向的指针，使其现在指向字符串中出现特定字符的位置。
因为数组降级为指针（并且指针可以被视为数组），如果您有以下情况，您将经常看到指向指针的指针：
指向数组的指针。这是上述情况的概括，因为“字符串”（无论如何都是 C 风格的字符串）实际上只是一个
chars数组。指针数组。例如，您可能有一个指向对象的指针数组，允许多态性，或者一个指针数组来选择存储在另一个集合中的对象。
数组数组。同样，数组降级为指针，所以这是上述情况的一个特例。这通常用于所谓的“锯齿状”阵列（与矩形相反）。
回答by BC.
You can interpret it literally -- pointer to a pointer
你可以从字面上解释它——指向一个指针的指针
回答by Tony the Pony
int **vardeclares a pointer to a pointer**varreferences the content of a pointer, which in itself points to a pointer
int **var声明一个指向指针的指针**var引用一个指针的内容，它本身指向一个指针
回答by Martin Beckett
One common use is that it allows a function to set the pointer to null.
So free(pointer) frees up the memory allocated to pointer but leaves the pointer dangerously pointing at the free memory.
Instead declare a my_free(**pointer) and call my_free(&pointer) so my_free() can set the pointer to null after freeing it.
一个常见的用途是它允许函数将指针设置为空。
所以 free(pointer) 释放了分配给指针的内存，但让指针危险地指向了空闲内存。
而是声明一个 my_free(**pointer) 并调用 my_free(&pointer) 以便 my_free() 可以在释放指针后将指针设置为 null。
