C ++中的new和malloc之间的区别 [英] differences between new and malloc in c++

问题描述

#include <iostream>
#include <cstdlib>
using namespace std;

class Box {
   public:
      Box() {
         cout << "Constructor called!" <<endl;
      }
      void printer(int x)
    {
        cout<<x<<" printer"<<endl;
    }

      ~Box() {
         cout << "Destructor called!" <<endl;
      }

};

int main( ) {
    Box* myBoxArray = new Box[4];

    Box* myBoxArray2 = (Box*)malloc(sizeof(Box[4]));
    myBoxArray2->printer(23);
    *myBoxArray2;
    *(myBoxArray2).printer(23);

   return 0;
}

问题很简单，当我使用'new'时，构造函数被打印出来，但是当我简单地反引用myBoxArray2的指针时，构造函数不会被打印，功能printer也不会被打印. 也是为什么当我使用->时功能打印机运行，但是当我使用等效的*(myBoxArray2).printer(23)

the problem simply is that when i use 'new' the constructor is printed out but when i simple derefrence the pointer to myBoxArray2 the constructor is not printed and neither is the funtion printer printed. Also why is it that when i use -> the funnction printer runs but not when i use the equivalent *(myBoxArray2).printer(23)

推荐答案

malloc仅分配内存 ，它不会调用会使对象保持不确定状态的构造函数.

malloc allocates memory only, it doesn't invoke constructors which can leave objects in an indeterminate state.

在C ++中，您几乎切勿使用malloc，calloc或free.并且尽可能避免使用new和new[]，而是使用对象实例或实例向量.

In C++ you should almost never use malloc, calloc or free. And if possible avoid new and new[] as well, use object instances or vectors of instances instead.

对于第二个问题(与第一个问题确实无关)，由于.选择运算符具有较高的

As for your second question (which is really unrelated to the first), *(myBoxArray2).printer(23) is wrong since the the . selection operator have higher precedence than the dereference operator *. That means first of all that you use the . member selector on a pointer which is invalid, and that you attempt to dereference what printer returns which is also wrong since it doesn't return anything.

您要(*myBoxArray2).printer(23)(请注意，星号的位置在括号内的 内)，它与myBoxArray2->printer(23)完全相同..

You want (*myBoxArray2).printer(23) (note the location of the asterisk is inside the parentheses), which is exactly the same as myBoxArray2->printer(23).

还请注意，myBoxArray2->printer(23)与myBoxArray2[0].printer(23)相同.

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