改变函数中的指针，c [英] changing pointers in functions, c

问题描述

我会提前说我在询问指针的指针，所以我的话可能有点含糊，但尽量留在我身边:)

我试图了解更改作为参数传递的指针如何在函数范围之外生效.到目前为止，这是我的理解:如果将指针作为参数传递给函数，我只能更改指针指向的内容，而不能更改指针本身(我说的是将在作用域外生效的更改的功能，正如我所说).如果我想改变指针指向的东西，我必须传递一个指向指针的指针.

到目前为止我是对的吗?另外，我注意到当我有一个包含一些指针的结构时，如果我想初始化这些指针，我必须将 struct 作为指向指针的指针传递给初始化函数.这是出于同样的原因吗?

解决方案

你说得对，但如果你已经分配了结构体，那么你可以传递一个指向它的指针.但是，如果函数分配了结构体，那么要么使用函数返回来收集新分配结构体的值，要么传入一个指向参数列表中的指针的指针.

(我手头没有 c 编译器，但我试着写了一些例子).

• 你已经分配了指针

<前>int main() {struct x *px = malloc(...);初始化(像素)；}void intix(struct x* px){像素-> ....}

• 分配指针的函数

<前>int main() {结构 x *px = initx();}结构 x* intix(){struct x *px = malloc(...);像素-> ....返回像素；}

• 分配指针的函数

<前>int main() {结构 x *px;initx(&px);}void intix(struct x** ppx){struct x *px = malloc(...);像素-> ....*ppx = px;}

I'll say in advance that I'm asking about pointers to pointers, so my words may be a bit vague, but try to stay with me :)

I am trying to understand how changing a pointer passed as an argument takes effect outside the scope of the function. So far this is what I understand: if a pointer is passed as an argument to a function, I can only change what the pointer points to, and not the pointer itself (I'm talking about a change that will take effect outside the scope of the function, as I said). And if I want to change what the pointer is pointing to, I have to pass a pointer to a pointer.

Am I right so far? Also, I've noticed that when I have a struct that holds some pointers, if I want to initialize those pointers I have to pass the struct to the initialization function as a pointer to pointer. Is this for the same reason?

解决方案

You are right in the first bit but if you've allocated the struct then you can pass a pointer to it. However, if the function allocated the struct, then either you use the function return to collect the value of the newly allocated struct or you pass in a pointer to a pointer in the parameter list.

(I've not got a c compiler to hand but I've tried to write some examples).

• You've allocated the pointer

    int main() {
       struct x *px = malloc(...);
       initx(px); 
    }

    void intix(struct x* px){
        px-> ....
    }
 

• The function allocated the pointer

     int main() {
       struct x *px = initx(); 
     }

     struct x* intix(){
        struct x *px = malloc(...);
        px-> ....
        return px;
     }
 

• The function allocated the pointer

     int main() {
       struct x *px;
       initx(&px); 
     }

     void intix(struct x** ppx){
        struct x *px = malloc(...);
        px-> ....
        *ppx = px;
     }
 

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