global/class placement new

#include <iostream>#include <typeinfo>using namespace std;class Base{public:    Base():a(0){};    ~Base(){};    void func(){}    int a;};class Derived : public Base{public:    Derived(){ cout << "Constructing Derived Object." << endl; }    virtual void func(){    cout << "in virtual func" << endl;    }    static void * operator new(size_t s, void* p){    cout << "Placement new in Derived" << endl;    return ::operator new(s, p); // call global placement new, 存在預設的全域 placement new    }}; int main(){    Derived d;    Base b;    void * p = ::operator new(sizeof(Derived));    new(p) Derived(); // call Derived's placement new，成功識別類自訂的placement new    Derived *pd = static_cast<Derived *>(p);    cout << sizeof(b) << endl << sizeof(d) << endl;    pd->func();    return 0;}

輸出

Constructing Derived Object.Placement new in DerivedConstructing Derived Object.48in virtual func