Function Returning a Pointer Generated From a Vector of Const Pointers

Question

I am having trouble finding a solution to my homework. Here's what I am trying to do: I have a list of objects stored into a vector. The vector, called "AvailableObjects" is declared globally.

vector<const Object*> AvailableObjects;

...

void       read_obj_file(const char* filename){
ifstream infile (filename, ios::in);

while(!infile.eof()) {
        char name[20];
        int attribute;
        int size = 0;
        infile >> name >> attribute >> size;
        AvailableObjects.push_back(new Object(name, attribute, size));
        }
 infile.close();
 return;
 }

After reading the objects, I need to write a function to generate a single object, and push it on to a stack of objects available to the user.

Object*     generate_object(){
        return AvailableObjects.at(rand_in_range(1,AvailableObjects.size()));
        }

The code above is what I wanted to use. I need to randomly select one of the objects stored in the vector, and return a pointer of that object back to whatever called the function. However, this cannot be done as the objects in the vector are const Object*, not simple Object*. This is a homework assignment, so I cannot modify the const values, they prototypes must remain the same. Lastly, I will share the object class. It has a constructor dedicated to creating a new object when passed a const Object*, but I cannot make the constructor work as it is intended.

/**
 * object.h
 *
 * Objects are immutable (read-only) once instantiated.
 */
#ifndef OBJECT_H
#define OBJECT_H

#include<string>
    using std::string;

class object{
    private:
    string _name;
    int    _attribute;
    int    _size;

    public:
    // Constructor
    Object(string name, int attribute, int size){
        _name  = name;
        _attribute = attribute;
        _size  = size;
    }
    Treat(const Treat& t){
        _name  = t._name;
        _attribute = t._attribute;
        _size  = t._size;
    }
    // Accessors
    string name() const {return _name;}
    int    attribute()const {return _attribute;}
    int    size() const {return _size;}
    };

    #endif

And here, also, is a function displayed throughout the code that chooses a random number in a particular range.

    int rand_in_range(int low, int high){
        init_rand();
        high < low ? throw "Invalid range." : 0 ;
        int modbase = ((high+1) - low);
        int ans = 0;
        ans = rand() % modbase + low;
        return ans;
        }

Thanks for any response, I'll be actively watching this, so, should anyone have any questions, I'll be happy to reply. Again, just to summarize, I need help to get my generate_object function to return an Object* using the vector of const Object* available to me.

side note: class object and a constructor called Object and copy constructor called Treat won't even begin to compile correctly. Please post valid code if you want answers concerning it. — WhozCraig
– WhozCraig, Commented Oct 30, 2012 at 23:37

cgmb · Accepted Answer · 2012-10-30 23:35:43Z

1

Vectors are zero-indexed, so the valid range of AvailableObjects.at is 0 to AvailableObjects.size()-1.

Presuming Treat(const Treat& t) should be Object(const Object& t) and that you've made a mistake in transcribing, then it does not take a const Object* as you said. Since it takes a const reference, not a const pointer, you must dereference your pointer. For example, if we wanted to make a deep copy of the fifth object in AvailableObjects we'd do:

int index = 4; // 0, 1, 2, 3, 4... thus index 4 refers to the fifth item.
const Object* object = AvailableObjects.at(index);
Object* clone = new Object(*object);

answered Oct 30, 2012 at 23:35

cgmb

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Comments

Mahmoud Fayez · Accepted Answer · 2012-10-30 22:46:01Z

1

First the range should start from zero not one. Second you can remove the const by type casting it like the following link

http://msdn.microsoft.com/en-US/library/bz6at95h(v=vs.80).aspx

answered Oct 30, 2012 at 22:46

Mahmoud Fayez

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