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Container and storing non pointers


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#1 rpiller   Members   -  Reputation: 683

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Posted 21 December 2013 - 02:04 PM

I'm confused as to why the object in my containers isn't reflecting the changes I make when I get the object by ref and call a function that changes that objects internal data member value. I know that I can store pointers in the container and return pointers from the Storage class but I was curious as to why returning references doesn't work the way I'm thinking it would.

 

 

class AI : public Component
{
private:
    int testing;
public:
    AI() { testing = 2;}
    AI(int id) : Component(id) { testing = 5;}
    virtual void Update()
    {
    }
 
    void SetTesting(int v) { testing = v; }
    int GetTesting() { return testing; }
};
 
//--------------------------------
 
template<class T>
class Storage
{
private:
    map<int, T> data;
public:
    T& Add(Object& obj)
    {
        int id = obj.GetID();
 
        // make sure this object can only have 1 of this type
        if(data.find(id) == data.end())
        { 
            data[id] = T(obj.GetID());
        }
 
        return data[id];
    }
 
    T& Get(Object& obj)
    {
        return data[obj.GetID()];
    }
 
    void Update()
    {
        map<int, T>::iterator iter;
 
        for(iter = data.begin(); iter != data.end(); ++iter)
            (*iter).second.Update();
    }
};
 
//--------------------------------
 
int main()
{
    Storage<AI> ai;
 
    Object o1;
 
    AI a1 = ai.Add(o1);    // get the reference of the AI object
    a1.SetTesting(10);    // change it's internal member variable to 10
 
    // however at this point if I mouse over ai it's still showing the internal variable as 5. I would think that because .Add() returned a reference to the object in the container that any changes I make would be done to that object in the container
 
    return 0;
}


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#2 KulSeran   Members   -  Reputation: 2537

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Posted 21 December 2013 - 02:09 PM

You didn't grab a reference..

AI a1 = a1.Add(o1);

is making a copy.  While "add" returns a reference value, since a1 isn't also a reference value it will use the copy-constructor AI(const AI &other) to construct a copy called a1.

 

You want to do:

AI &a1 = a1.Add(o1);

to have a reference. Since that way, a1 is of the type "reference to an AI", and can hold the exact reference that "add" returned.


Edited by KulSeran, 21 December 2013 - 02:12 PM.


#3 Paradigm Shifter   Crossbones+   -  Reputation: 5372

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Posted 21 December 2013 - 02:10 PM

You can't change the key value for an object in a map, that would invalidate it. If you want to change the key, erase the object change the key and put it back in again.


"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

#4 rpiller   Members   -  Reputation: 683

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Posted 21 December 2013 - 02:50 PM

Thanks KulSeran, I missed that, or assumed since the return type had & in it that it would either give me an error or auto convert on the variable I was assigning it to.

 

 

Why is it when I step through the adding of my object to the container the ctor of AI with the parameter is called first, then the default ctor called? I assume my T(obj.GetID()) calls the one with the parameter and the container itself is calling the default one, and then is my explicit object getting copied into the container? If so, how deep is this coping because it seems to auto copy my int variable but what if I had other objects being created inside AI, would it copy all that too? This maybe seems to be the biggest downfall to storing non pointers. I originally was thinking not using pointers in the container would allocate on the stack but seems containers allocate the actual object it contains on the heap anyway, so to avoid copying and 2 ctors getting called seems like storing pointers would be the way to go anyway.


Edited by rpiller, 21 December 2013 - 02:54 PM.


#5 Brother Bob   Moderators   -  Reputation: 8194

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Posted 21 December 2013 - 04:22 PM

Why is it when I step through the adding of my object to the container the ctor of AI with the parameter is called first, then the default ctor called? I assume my T(obj.GetID()) calls the one with the parameter and the container itself is calling the default one, and then is my explicit object getting copied into the container? If so, how deep is this coping because it seems to auto copy my int variable but what if I had other objects being created inside AI, would it copy all that too? This maybe seems to be the biggest downfall to storing non pointers. I originally was thinking not using pointers in the container would allocate on the stack but seems containers allocate the actual object it contains on the heap anyway, so to avoid copying and 2 ctors getting called seems like storing pointers would be the way to go anyway.

A deep copy of an object would typically deep copy all its members as well. If the members also have members, they would be deep copied as well. This goes on recursively for as long as members have other members themselves. And assuming your copy constructor/assignment don't do things like reference counted resource management or copy-on-write magic and such by "virtually" copying the resource instead, of course.

 

You seem to think that not using pointers means the object ends up on the stack. This is not correct. An object ends up where it is allocated. If you allocate an object with new, then the members of that object is also allocated as a part of the parent object. In this case, the map allocates its copy of the value you put into it dynamically, so the object will not be on the stack.

 

Storing a container of values does require copying them into the container, but, especially for linear containers such as the vector, this is often vastly superior to pointers as you gain cache locality since all objects are stored linearly in memory. Storing pointers means another level of pointer indirection any time you want to access an object in the container, and that can be expensive. You often add an object just once, but access it many times. If you then trade efficiency in adding the object for accessing it, you have made the wrong decision if copying it is not too expensive compared to how often you access it.






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