What happened if i delete a pointer which was not allocated dynamically?

朱雀 2022-07-24 05:18 120阅读 0赞

new/delete new\[\]/delete\[\]实在是老生常谈了，成对的出现就可以了：

#include <iostream> // std::cout
    #include <new> // ::operator new
    
    struct MyClass {
      int data[100];
      MyClass() {
       std::cout << "constructed [" << this << "]\n";}
    };
    
    int main () {
    
      std::cout << "1: ";
      MyClass * p1 = new MyClass;
          // allocates memory by calling: operator new (sizeof(MyClass))
          // and then constructs an object at the newly allocated space
    
      std::cout << "2: ";
      MyClass * p2 = new (std::nothrow) MyClass;
          // allocates memory by calling: operator new (sizeof(MyClass),std::nothrow)
          // and then constructs an object at the newly allocated space
    
      std::cout << "3: ";
      new (p2) MyClass;
          // does not allocate memory -- calls: operator new (sizeof(MyClass),p2)
          // but constructs an object at p2
    
      // Notice though that calling this function directly does not construct an object:
      std::cout << "4: ";
      MyClass * p3 = (MyClass*) ::operator new (sizeof(MyClass));
          // allocates memory by calling: operator new (sizeof(MyClass))
          // but does not call MyClass's constructor
    
      delete p1;
      delete p2;
      delete p3;
    
      return 0;
    }

// operator delete[] example
    #include <iostream> // std::cout
    
    struct MyClass {
      MyClass() {
       std::cout <<"MyClass constructed\n";}
      ~MyClass() {
       std::cout <<"MyClass destroyed\n";}
    };
    
    int main () {
      MyClass * pt;
    
      pt = new MyClass[3];
      delete[] pt;
    
      return 0;
    }

我们在写C++程序的时候，非常的小心，很怕会遇到内存泄露的问题。  
所以，只记得要delete或delete\[\]指针，然后赋值为nullptr。

但是往往会为我们的过于谨慎付出代价，当我们delete非new产生的指针会怎样呢？

看代码：

#include <iostream>
    using namespace std;
    
    int main()
    {
        int *a;
        int b = 5;
        a = &b;
        cout << *a << endl;
        delete a;
        return 0;
    }

上面的程序会崩溃，那下面的呢，居然没问题？

int main()
    {
        int *a = 0;
        delete a;
        return 0;
    }

原因：

第一段代码会导致不确定的行为，原因就是你delete了非new指针。

第二段代码表面上没问题，是因为delete一个null指针不会有什么危害。

所以，切记  
**不要delete a pointer which was not allocated dynamically**