memory - 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

Because malloc doesn't call the class's constructor, and doesn't know anything about any particular alignment requirements it might have. If you need to use malloc (not recommended), take a look at placement new (assuming you don't want to overload the regular new for some reason).