C++ operator new placement new

一、整体代码

       01.cpp

#include<iostream>#include <stdlib.h>using namespace std;class Test{public:   Test(int n) : n_(n)   {       cout<<"Test(int n) : n_(n)"<<endl;   }   Test(const Test& other)   {       cout<<"Test(const Test& other)"<<endl;   }   ~Test()   {       cout<<"~Test()"<<endl;   }   void *operator new(size_t size)//普通情况调用，标注为0   {       cout<<"void* operator new(size_t size)"<<endl;       void* p = malloc(size);       return p;   }   void operator delete(void* p)   {       cout<<"void operator delete(void* p)"<<endl;       free(p);   }   void operator delete(void* p, size_t size)//如果上面那个和这个delete都有，那么调用上面的。   {       cout<<"void operator delete(void* p, size_t size)"<<endl;       free(p);   }   void* operator new(size_t size, constchar* file, long line)//另一种情况调用，标注为1   {       cout<<file<<":"<<line<<endl;       void* p = malloc(size);       return p;   }   void operator delete(void* p, constchar* file, long line)   {       cout<<file<<":"<<line<<endl;       free(p);   }   void* operator new(size_t size, void* p)//另一种情况调用，标注为2   {       cout<<"run"<<endl;       return p;   }   void operator delete(void*, void*)   {       cout<<"destory"<<endl;   }   int n_;};void* operator new[](size_t size)//全局非Test类调用此方法，标注为3{   cout<<"global void* operator new[](size_t size)"<<endl;   void* p = malloc(size);   return p;}void operator delete[](void* p){   cout<<"global void operator delete[](void* p)"<<endl;   free(p);}int main(void){   Test* p1 =new Test(100);    // new operator = operator new + 构造函数的调用 调用标注0   delete p1;                  //delete = 析构函数的调用+operator delete   char* str =new char[100]; //调用标注3   delete[] str;   char chunk[10];   Test* p2 =new (chunk) Test(200);    //operator new(size_t, void *_Where)+构造函数的调用                                       // placement new，不分配内存+ 构造函数的调用                                       //调用标注2   cout<<p2->n_<<endl;   p2->~Test();                        // 显式调用析构函数，因为没有分配内存，所以直接调用析构函数   return0;}

二、解释

       1、 Test* p1 = new Test(100); 

              delete p1;

             结果如下：

             void* operator new(size_t size)
             Test(int n) : n_(n)
            ~Test()
             void operator delete(void* p)

        

        2、char * str = new char[100];

              delete[] str;

              结果如下：

               global void* operator new[](size_t size)
               global void operator delete[](void* p)

        3、char chunk[10];

              Test* p2 =new (chunk) Test(200);

              cout<<p2->n_<<endl;
              p2->~Test()

              结果如下：

              run
             Test(int n) : n_(n)
             200
             ~Test()