Thinking more...

#include <iostream>

class MyBase {
  protected:
    int _id;

  public:
    MyBase():_id(0) {}
    MyBase( int id ):_id(id) {}
    int getId() {return _id; }
};

class MyClass: public MyBase {
  public:
    MyClass() {}
    MyClass( int id ):MyBase( id ) {}
};

int myclass_mem[ sizeof(MyClass)/sizeof(int) ];

using namespace std;

MyBase* test()
{
  return new (myclass_mem)MyClass( 100 );// 這裡 (1)
  //return new MyClass( 100 ); // (2)
}

int
main( int argc, char* argv[] )
{
  int len = sizeof( myclass_mem );
  cout << "The size of myclass_mem is " << len << endl;
  cout << "content of myclass_mem" << endl;
  for( int i=0; i<len; i++ )
    cout << myclass_mem[i] << " ";
  cout << endl;

  MyBase* base = ::test();
  cout << base->getId() << endl;

  cout << "content of myclass_mem" << endl;
  for( int i=0; i<len; i++ )
    cout << myclass_mem[i] << " ";
  cout << endl;
}