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

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