Question

Just a simple question. As the title suggests, I’ve only used the “new” operator to create new instances of a class, so I was wondering what the other method was and how to correctly use it.

Answer 1

You can also have automatic instances of your class, that doesn’t use new, as:

class A{};

//automatic 
A a;             

//using new
A *pA = new A();

//using malloc and placement-new
A *pA = (A*)malloc(sizeof(A));
pA = new (pA) A();

//using ONLY placement-new
char memory[sizeof(A)];
A *pA = new (memory) A();

The last two are using placement-new which is slightly different from just new. placement-new is used to construct the object by calling the constructor. In the third example, malloc only allocates the memory, it doesn’t call the constructor, that is why placement-new is used to call the constructor to construct the object.

Also note how to delete the memory.

  //when pA is created using new
  delete pA;

  //when pA is allocated memory using malloc, and constructed using placement-new
  pA->~A(); //call the destructor first
  free(pA); //then free the memory

  //when pA constructed using placement-new, and no malloc or new!
  pA->~A(); //just call the destructor, that's it!

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To know what is placement-new, read these FAQs:

• What is “placement new” and why would I use it? (FAQ at parashift.com)
• placement new (FAQ at stackoverflow.com)