Why do we need placement new operator?

Question

I know that we need placement new operator when memory for an object is to be allocated at a specfic memory location. e.g.

int* MemoryBuffer = malloc(sizeof(int)*10);
MyClass* Object = new (MemoryBufer) Myclass;

Can't we simply do like this

MyClass* Object = reinterpret_cast<MyClass*>(MemoryBuffer);

Object will point to the memory allocated by malloc above. Why do we need placement new operator, does it do the same thing or there is any difference?

The cast just points to uninitialized memory so that's not an alternative. Keep thinking about it and you'll inevitably find a use for it. — Hans Passant, Mar 25 '16 at 10:41
Thanks! I missed the point that after new, constructor will be invoked. — Akshat, Mar 25 '16 at 10:43

score 2 · Accepted Answer · answered Mar 25 '16 at 10:43

If you write:

    auto MemoryBuffer1 = malloc(sizeof(Myclass)); 
    Myclass *pObject1 = new (MemoryBuffer1) Myclass;

    auto MemoryBuffer2 = malloc(sizeof(Myclass));
    Myclass *pObject2 = reinterpret_cast<Myclass*>(MemoryBuffer2);

Then *pObject1 is a properly constructed object, and the constructor will have been called. *pObject2 is just a pointer to a random collection of bytes, and is not a valid object.

zoska · Answer 2 · 2016-03-25T11:05:00.767

0

The first one (placement new) calls constructor of MyClass and second one (reinterpret cast) doesn't.

edited Mar 25 '16 at 11:05

answered Mar 25 '16 at 10:41

zoska

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Why do we need placement new operator?

2 Answers2