convert default ctor to anthoer ctor with placement new

you can define T (that is a class),defined two constructor and one for destructor.but you use new T for obtianning this class.if you want to rebuild constructor to other constructor without new memory for that operation,you can use replacement new operation,that can solve this probelem.

/*************************************************************************
    > File Name: t.cc
  > Author: perrynzhou
  > Mail: [email protected]
  > Created Time: Fri 18 Aug 2017 06:33:20 PM AKDT
 ************************************************************************/

#include 
#include 
using namespace std;
class T {
public:
    T()
        : m_data(0)
    {
        cout << "default.ctor.this =" << this << ", m_data =" << m_data << endl;
    }
    T(int _data)
        : m_data(_data)
    {
        cout << "ctor.this =" << this << ", m_data=" << m_data << endl;
    }
    ~T() { cout << "dtor.this =" << this << ", m_data=" << m_data << endl; }

private:
    int m_data;
};
int main(void)
{
    int size = 4;
    T* tes = new (std::nothrow) T[size];
    assert(tes != NULL);
    cout << "new T[" << size << "] addr=" << tes << endl;
    T* tmp = tes;
    cout << "tmp addr=" << tmp << endl;
    for (int i = i; i < size; i++) {
        new (tmp++) T(i + 1);
    }
    delete[] tes;
    return 0;
}

replacement new can be converted to this:

// statment
new(temp++)T(i)


//gcc complier can convert as follow:
  T *tes;
  try {
      void *mem  =  operator new(sizeof(T),temp);
      tes = static_cast(mem);
      tes->T::T(i);
  }  cache(std::bad_alloc){}

//operator new 
void *opertor new(size_t,void *alloc)
{
    return alloc;
}

T *obj = new T  ---> { 
                                        T t= (T *)operator new(sizeof(T));   -->{ malloc(sizeof(T);}
                                        new(T) T();
                           }

delete obj          ---> {
                                      obj->~T();
                                      opertor delete(obj);  --->{free(obj);
                            }

running this program,you can show some as follow:

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