c++ casting struct pointer to the type value

Question

This may be a stupid question but I'm learning, I'm having trouble with my code:

I'm having problems at this line:

fb->AddEntity(m); // M Needs to be the type Item not Item*

What I need is the object of the value for that type, not the pointer, because I need to pass it to the AddEntity function, How do I cast it?

#include "Handler.h"

FatBoy *fb;

void Entity::Interact()
{
};

void OnNewPacketReceived(BYTE *data, int Length)
{
    switch (data[0])
    {
        case 0x07:
            {
                short length = (short)((data[1] << 8) + data[2]);
                int Index = 0;

                for (int i = 0; i < length; i++)
                {
                    Entity ent;
                    ent.X = (USHORT)((data[Index] << 8) + data[Index + 1]);
                    ent.Y = (USHORT)((data[Index + 2] << 8) + data[Index + 3]);
                    ent.Serial = (UINT)((data[Index + 4] << 24) + (data[Index + 5] << 16) 
                        + (data[Index + 6] << 8) + data[Index + 7]);
                    ent.SpriteID = (USHORT)((data[Index + 8] << 8) + data[Index + 9]);

                    if (ent.SpriteID > 0x8000 && ent.SpriteID < 0x9000)
                    {
                        Item *m = dynamic_cast<Item*>(&ent); 
                        m->UNKNOWN1 = 0x00;
                        m->UNKNWON2 = 0x0000;
                        m->Interact();

                        fb->AddEntity(m); // M Needs to be the type Item not Item*
                    }
                    else
                    {
                        Monster *m = dynamic_cast<Monster*>(&ent); 
                        m->UNKNOWN = 0x00000000;
                    }
                }
            } break;
        default: break; 
    }
}

void FatBoy::AddEntity(Entity ent)
{
    this->Entities.push_back(ent);
}

Those dynamic_casts look scary and will both return null pointers, unless Entity inherits from both Monster and Item - but in that case, they're unnecessary (and it's likely that the inheritance is the other way around). You can't alter an object's class by casting it. — molbdnilo
– molbdnilo, Commented Dec 21, 2012 at 15:46

simonc · Accepted Answer · 2012-12-21 14:40:00Z

5

Use *m if you have a pointer and want to pass by value or by reference

fb->AddEntity(*m);

answered Dec 21, 2012 at 14:40

simonc

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1 Comment

simonc Over a year ago

Glad to have helped. If you look at Ben Voigt's answer, you'll find a further, optional, suggestion which would improve the efficiency of your code

Ben Voigt · Accepted Answer · 2012-12-21 14:40:12Z

5

To get the object from a pointer, use the dereference operator *.

So you would say

fb->AddEntity(*m);

In fact, every time you use m->member, that is exactly the same as (*m).member.

On the other hand, AddEntity should probably not be passing by value (it makes unnecessary copies). Consider

void FatBoy::AddEntity(const Entity& ent)

instead.

answered Dec 21, 2012 at 14:40

Ben Voigt

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