Overallocating with new/delete

Question

Using malloc and free, it is easy to allocate structures with extra data beyond the end. But how do I accomplish the same with new/

Answer 1

If I were you, I'd use placement new and an explicit destructor call instead of delete.

template< typename D, typename T >
D *get_aux_storage( T *x ) {
    return reinterpret_cast< D * >( x + 1 );
}

int main() {
    char const *hamburger_identity = "yum";
    void *hamburger_room = malloc( sizeof( Hamburger )
                                   + strlen( hamburger_identity ) + 1 );
    Hamburger *hamburger = new( hamburger_room ) Hamburger;
    strcpy( get_aux_storage< char >( hamburger ), hamburger_identity );
    cout << get_aux_storage< char const >( hamburger ) << '\n';

    hamburger->~Hamburger(); // explicit destructor call
    free( hamburger_room );
}

Of course, this kind of optimization should only be done after profiling has proven the need. (Will you really save memory this way? Will this make debugging harder?)

There might not be a significant technical difference, but to me new and delete signal that an object is being created and destroyed, even if the object is just a character. When you allocate an array of characters as a generic "block," it uses the array allocator (specially suited to arrays) and notionally constructs characters in it. Then you must use placement new to construct a new object on top of those characters, which is essentially object aliasing or double construction, followed by double destruction when you want to delete everything.

It's better to sidestep the C++ object model with malloc/free than to twist it to avoid dealing with data as objects.

Oh, an alternative is to use a custom operator new, but it can be a can of worms so I do not recommend it.

struct Hamburger {
  int tastyness;
public:
  char *GetMeat();
  static void *operator new( size_t size_of_bread, size_t size_of_meat )
      { return malloc( size_of_bread + size_of_meat ); }
  static void operator delete( void *ptr )
      { free( ptr ); }
};

int main() {
    char const *hamburger_identity = "yum";
    size_t meat_size = strlen( hamburger_identity ) + 1;
    Hamburger *hamburger = new( meat_size ) Hamburger;
    strcpy( hamburger->GetMeat(), hamburger_identity );
    cout << hamburger->GetMeat() << '\n';
}