普通new和placement new的重载

对于自定义对象，我们可以重载普通new操作符，这时候使用new Test()时就会调用到我们重载的普通new操作符。

示例程序：

 #include <iostream>
 #include <cstdlib>

 using namespace std;

 class Test
 {
 public:
     Test()
     {
         cout << "Test()" << endl;
     }

     void* operator new(unsigned int size)
     {
         void* ret = malloc(sizeof(int) * size);

         cout << "normal new" << endl;

         return ret;
     }

 };

 int main()
 {
     Test* t = new Test();

     Test t2;

     return ;
 }

执行结果如下：

调用placement new，程序如下：

 #include <iostream>
 #include <cstdlib>

 using namespace std;

 class Test
 {
 public:
     Test()
     {
         cout << "Test()" << endl;
     }

     void* operator new(unsigned int size)
     {
         void* ret = malloc(sizeof(int) * size);

         cout << "normal new" << endl;

         return ret;
     }

 };

 int main()
 {
     Test* t = new Test();

     Test* t1 = new((void*)t)Test();

     Test t2;

     return ;
 }

编译结果如下：

提示我们没有对应的函数，也就是placement new没有重载。

更改程序：

 #include <iostream>
 #include <cstdlib>

 using namespace std;

 class Test
 {
 public:
     Test()
     {
         cout << "Test()" << endl;
     }

     void* operator new(unsigned int size)
     {
         void* ret = malloc(sizeof(int) * size);

         cout << "normal new" << endl;

         return ret;
     }

     void* operator new(unsigned int size, void* loc)
     {
         cout << "placement new" << endl;
         return loc;
     }

 };

 int main()
 {
     Test* t = new Test();

     Test* t1 = new((void*)t)Test();

     Test t2;

     return ;
 }

结果如下：

再次给出一个测试程序：

 #include <iostream>
 #include <cstdlib>

 using namespace std;

 class Test
 {
 public:
     Test()
     {
         cout << "Test()" << endl;
     }

     void* operator new(unsigned int size)
     {
         void* ret = malloc(sizeof(int) * size);

         cout << "normal new" << endl;

         return ret;
     }

     void* operator new(unsigned int size, void* loc)
     {
         cout << "placement new" << endl;
         return loc;
     }

 };

 int main()
 {
     Test* t = new Test();

     Test* t1 = new((void*)t)Test();

     Test t2;

     Test* t3 = new((void*)&t2)Test();

     int a = ;

     int* p = new((void*)&a)int();

     cout << "a = " << a << endl;

     return ;
 }

运行结果如下：

可以看到普通内置类型可以直接使用placement new。