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:
    Derive         


        

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.