返回一个指针结构 [英] Returning a struct pointer

问题描述

假设我有下面的结构和函数返回一个指针：

  typedef结构{
  INT NUM;
  无效* NUMS;
  INT大小;
} MYSTRUCT;MYSTRUCT * MYSTRUCT（INT NUM，INT大小）
{
   //是下面的正确吗？有没有更有效的方法？
   MYSTRUCT MYSTRUCT;
   mystruct.num = NUM​​;
   mystruct.size =大小;
   mystruct.nums =的malloc（NUM *的sizeof（大小））;
   MYSTRUCT *我的;
   *我= MYSTRUCT;
   返回我的;
}
 

我想用上面的函数定义任何MYSTRUCT指针。我要声明一个变量MYSTRUCT，定义MYSTRUCT的属性，指针分配给它，并返回指针或通过指针定义MYSTRUCT财产的属性立刻？

解决方案

  

我应该声明一个变量MYSTRUCT，
  限定MYSTRUCT的属性，
  一个指针分配给它，并返回
  指针

肯定不是，因为变量在函数中定义（在自动存储类）将消失函数退出，你会返回一个悬摆指针。

您可以的接受的一个指针 MYSTRUCT （主叫方的责任来分配），并填写好;或者，你可以使用的malloc 以创建一个新的（调用者负责释放它，当它完成）。第二个选项，至少可以让你保持函数签名，你似乎热衷于：

  MYSTRUCT * MYSTRUCT（INT NUM，INT大小）
{
   MYSTRUCT * P =的malloc（sizeof的（MYSTRUCT））;
   ....
   回磷;
}
 

但通常较差的一 - 因为调用者必须有责任，无论如何，可以与第一选择还是去和可能获得的性能（如，因为它知道使用范围的调用者可以使用自动类实例为界）。

Suppose I have the following struct and function returning a pointer:

typedef struct {
  int num;
  void *nums;
  int size;
} Mystruct;

Mystruct *mystruct(int num, int size)
{ 
   //Is the following correct? Is there a more efficient way?
   Mystruct mystruct;
   mystruct.num = num;
   mystruct.size = size;
   mystruct.nums = malloc(num*sizeof(size));
   Mystruct *my;
   *my = mystruct;
   return my;
}

I want to define any Mystruct pointer using the above function. Should I declare a Mystruct variable, define the properties of Mystruct, assign a pointer to it, and return the pointer or define the properties of a mystruct property through a pointer immediately?

解决方案

Should I declare a Mystruct variable, define the properties of Mystruct, assign a pointer to it, and return the pointer

Definitely not, because the variable defined in the function (in "auto" storage class) will disappear as the function exits, and you'll return a dangling pointer.

You could accept a pointer to a Mystruct (caller's responsibility to allocate that) and fill it in; or, you can use malloc to create a new one (caller's responsibility to free it when it's done). The second option at least lets you keep the function signature you seem to be keen on:

Mystruct *mystruct(int num, int size)
{
   Mystruct *p = malloc(sizeof(MyStruct));
   ....
   return p;
}

but it's often an inferior one -- since the caller has to have responsibilities anyway, may as well go with the first option and potentially gain performance (if the caller can use an auto-class instance because it knows the scope of use is bounded).

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