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：
1 #include <iostream> 2 #include <cstdlib> 3 4 using namespace std; 5 6 class Test 7 { 8 public: 9 Test() 10 { 11 cout << "Test()" << endl; 12 } 13 14 void* operator new(unsigned int size) 15 { 16 void* ret = malloc(sizeof(int) * size); 17 18 cout << "normal new" << endl; 19 20 return ret; 21 } 22 23 24 }; 25 26 27 int main() 28 { 29 Test* t = new Test(); 30 31 Test t2; 32 33 34 return 0; 35 }
执行结果如下：
调用placement new，程序如下：
1 #include <iostream> 2 #include <cstdlib> 3 4 using namespace std; 5 6 class Test 7 { 8 public: 9 Test() 10 { 11 cout << "Test()" << endl; 12 } 13 14 void* operator new(unsigned int size) 15 { 16 void* ret = malloc(sizeof(int) * size); 17 18 cout << "normal new" << endl; 19 20 return ret; 21 } 22 23 24 }; 25 26 27 int main() 28 { 29 Test* t = new Test(); 30 31 Test* t1 = new((void*)t)Test(); 32 33 Test t2; 34 35 36 return 0; 37 }
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：
1 #include <iostream> 2 #include <cstdlib> 3 4 using namespace std; 5 6 class Test 7 { 8 public: 9 Test() 10 { 11 cout << "Test()" << endl; 12 } 13 14 void* operator new(unsigned int size) 15 { 16 void* ret = malloc(sizeof(int) * size); 17 18 cout << "normal new" << endl; 19 20 return ret; 21 } 22 23 void* operator new(unsigned int size, void* loc) 24 { 25 cout << "placement new" << endl; 26 return loc; 27 } 28 29 }; 30 31 32 int main() 33 { 34 Test* t = new Test(); 35 36 Test* t1 = new((void*)t)Test(); 37 38 Test t2; 39 40 41 return 0; 42 }
结果如下：
再次给出一个测试程序：
1 #include <iostream> 2 #include <cstdlib> 3 4 using namespace std; 5 6 class Test 7 { 8 public: 9 Test() 10 { 11 cout << "Test()" << endl; 12 } 13 14 void* operator new(unsigned int size) 15 { 16 void* ret = malloc(sizeof(int) * size); 17 18 cout << "normal new" << endl; 19 20 return ret; 21 } 22 23 void* operator new(unsigned int size, void* loc) 24 { 25 cout << "placement new" << endl; 26 return loc; 27 } 28 29 }; 30 31 32 int main() 33 { 34 Test* t = new Test(); 35 36 Test* t1 = new((void*)t)Test(); 37 38 Test t2; 39 40 Test* t3 = new((void*)&t2)Test(); 41 42 int a = 3; 43 44 int* p = new((void*)&a)int(10); 45 46 cout << "a = " << a << endl; 47 48 return 0; 49 }
运行结果如下：
可以看到普通内置类型可以直接使用placement new。