访问分配有malloc的C ++对象的成员？ [英] Accessing member of a C++ object allocated with malloc?

问题描述

以下代码在 top - > ... 与分段错误，不管 Node * 试图被推送到向量子节点。是否有人知道是什么原因导致的？

  struct Node 
 {
 string data; 
 struct Node * parent; 
 vector< struct Node *>儿童; 
}; 
 
 struct Node * push（string word，struct Node * top）
 {
 Node * temp =（struct Node *）malloc 
 temp  - > data = word; 
 temp  - > parent = top; 
 return temp; 
} 
 
 int main（）
 {
 Node * top = push（blah，NULL）; 
 Node * temp = NULL; 
 top  - > children.push_back（temp）; 
} 
  

解决方案

问题是 malloc 不会调用构造函数。

替换：

$ b $

  Node * temp =（struct Node *）malloc（sizeof（struct Node））; 
  

有：

 code> Node * temp = new Node; 
  

malloc 和 new （从C ++）都将分配所需的内存，但只有 new 会调用所需的构造函数，如C不使用它们。如果你不肯定你需要malloc，使用新的。

[Editor's note: I have edited the title to try to make this useful to others in the future. To give the answerers credit, this was just a "why does this not work?" question when they answered it!]

The following code crashes at the top -> ... line with a segmentation fault, regardless of what Node* is trying to be pushed onto the vector children. Does anyone know what might be causing this?

struct Node
{
    string data;
    struct Node* parent;
    vector<struct Node*> children;
};

struct Node* push(string word, struct Node* top)
{
    Node* temp = (struct Node*) malloc(sizeof(struct Node));
    temp -> data = word;
    temp -> parent = top;
    return temp;
}

int main()
{
    Node* top = push("blah", NULL);
    Node* temp = NULL;
    top -> children.push_back(temp);
}

解决方案

The problem is that malloc will not call constructors. And you're counting on the vector children to be constructed.

Replace:

Node* temp = (struct Node*) malloc(sizeof(struct Node));

With:

Node* temp = new Node;

malloc (from C) and new (from C++) will both allocate the memory you need, but only new will call the required constructors, as C doesn't use them. If you're not positive that you need malloc, use new.

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