Question

There is such code:

#include <iostream>

int main()
{
    int a;
    int* p = new (&a) int(2);
    std::cout << a << std::endl;
    // delete p; error BLOCK TYPE IS INVALID
    std::cin.get();
    return 0;
}

The output is:

2

Why is it possible to dynamically allocate memory on stack? (I thought that heap is the right place to do this). And, why does delete operator return error in this case, but new operator work?

Answer 1

int* p = new (&a) int(2);

This is called placement-new. It doesn’t allocate memory. It constructs the object in the same memory of a. In placement new, it’s the user who specifies the memory region where new operator constructs the object. In your code above, you specify the memory region by writing (&a) expression just after the new keyword. Since &a is not a dynamically allocated memory, you cannot delete it:

delete p; //runtime-error

It would give runtime error, it attempts to delete the memory where the variable a resides.

However, if you dynamically allocate the memory, then you can do delete it. Lets suppose, A is some class, then you should be doing this:

char *buffer = new char[sizeof(A)]; //allocate memory of sizeof(A);

///ASSUMPTION: the buffer is properly align as required by the type A
//use placement-new to construct an object at the specified memory region
A *pA = new (buffer) A(/*..parameters..*/); 

//...

//incorrect way to delete the memory!
//delete pA; //incorrect

//before deleting the memory you should be calling the destructor explicitly as
pA->~A(); //call the destructor explicitly - must do it

//now deallocate the memory as
delete []buffer;

This is simplest example of placement-new which explains the syntax only. But the story doesn’t end here; it is the beginning and to make it work properly, the memory pointed to by buffer has to be aligned properly for the object type, and in the above example, I simply assumed so. In the real code, you cannot make such dangerous assumption. Now read this FAQ:

• What is “placement new” and why would I use it?