什么是C ++中的**？ [英] What is ** in C++?

问题描述

我看过一些代码，以及我的编译器生成的一些错误，在变量之前有一个' ** '标记（例如** variablename unreferenced- - 或者什么，我不能记得完全手足无措）。我相当肯定这是与指针有关，如果我不得不猜测，它看起来像它尝试去引用两次。 ' ** '是相当不可取的。有人可以指点我的网站/文件或者有人在这里解释一下吗？

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?

谢谢。

伟大的反应。如果我可以添加，在某些情况下，有一个指针指针是有用的？你不应该只是使用原始指针，而不是创建另一个指针指向原始指针？

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?

推荐答案

**不是实际上只有指向指针的指针（如在声明中），但也是dereference（在语句中）的引用。

** is not actually only pointer to pointer (as in declaration), but is also the dereference of a dereference (in a statement).

&例如参考文献。更新一个指针类型的返回值：

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:

int alloc_foo(struct foo **foo_ret)
{
    *foo_ret = malloc(sizeof(struct foo));
    return 1; /* to indicate success; return value in foo_ret */
}

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