In C++, why is `new` needed to dynamically create an object rather just allocation?

Question

I've got this trivial class hierarchy:

class Base {
public:
    virtual int x( ) const = 0;
};

class Derived : public Base {
    int _x;
public:
    Derived( int x ) : _x(x) { }
    int x( ) const { return _x; }
};

If I use malloc to allocate an instance of Derived, and then try to access the polymorphic function x, program crashes (I get a segmentation fault):

int main( ) {
    Derived *d;
    d = (Derived*) malloc( sizeof(Derived) );
    *d = Derived( 123 );

    std::cout << d->x() << std::endl; // crash

    return 0;
}

Of course my actual application is a lot more complex (it's a sort of memory pool).

---

I'm pretty sure it's because of the way I allocate d: I didn't use new.

I know of placement new operator, which must be what I need, but I've never used it and have got some questions:

• why is my application crashing, if I don't use new?

  What does new actually do?

  Why can't I just use the assignment operator to assign the value of Derived( 123 ); to the memory area pointed by d?

• Would I need to use new also for non-polymorphic types?

  How about POD ones?

• On the C++Faq I linked above it says that the memory region passed to placement new must be aligned for the object I'm creating.

  I know what alignment is, but I don't know how to check the alignment needed for my class.

  malloc manual says:

  The malloc() and calloc() functions return a pointer to the allocated memory that is suitably aligned for any kind of variable.

  And I hope that the alignment needed for my class is the class size as returned by sizeof, so that any address in the form address_returned_by_malloc + i * sizeof(my_class) is suitable to allocate my objects.

  Are my hopes right?

Answer 1

Let's go down the line

1. why is my application crashing, if I don't use new?

Virtual table is corrupted.

The virtual table is stuck right after the allocated memory. when you new a class, the generated code will properly set up the vtable. However, malloc will not properly initialize the vtable

To see the virtual table, run g++ -fdump-class-hierarchy

Vtable for Derived
Derived::_ZTV7Derived: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI7Derived)
16    Derived::x

Class Derived
   size=16 align=8
   base size=12 base align=8
Derived (0x10209fc40) 0
    vptr=((& Derived::_ZTV7Derived) + 16u) <-- notice how this is part of the structure
  Base (0x10209fcb0) 0 nearly-empty
      primary-for Derived (0x10209fc40)

For a similar reason, without overloading operator=, the generated assembly code will only copy the data and not the vtable [again, the compiler only knows to copy the data, not the vtable]

If you want to see a pointer-based version with a valid vtable function:

Derived e(123);
d = &e;

1. Would I need to use new also for non-polymorphic types?

If you are using virtual functions, then yes, even for non-polymorphic types

1. I hope that the alignment needed for my class is the class size as returned by sizeof, so that any address in the form address_returned_by_malloc + i * sizeof(my_class) is suitable to allocate my objects.

Alignment is not an issue.