Question

I am creating a vector that contains pointers to a base class. In this vector I’m dynamically storing pointers to derived classes which contain some member variables, one of them being a string variable name.

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <string>
#include <cstdlib>

bool hasDirection = false;
bool hasDiameter = false;
int direction;
float diameter;
int starDimension = 0;
int animalDimension = 0;
int fishDimension = 0;
 class MovingObject
{
protected:
    std::string name;
    int direction;
    float diameter;
    int dimension;
    float movingSpeed;

public:
    std::string getName(){ return name;};
    int getDirection(){ return direction;};
    float getDiameter(){ return diameter;};
    float getMovingSpeed(){ return movingSpeed;};
    int getDimension(){ return dimension;};
    void setName(std::string v){ name = v;};
    void setDirection(int d){ direction = d;};
    void setDiameter(float f){ diameter = f;};
    void setMovingSpeed(float s){ movingSpeed = s;};
    void setDimension (int d){ dimension = d;};
    virtual void PrintContents()=0;
};

static std::vector<MovingObject*> data;

class starObject : public MovingObject
{
public:
    void PrintContents()
    {
        std::cout << "(" << getName() << "," << getDiameter() << "," << getDirection() << ")";
    }
};

class animalObject : public MovingObject
{
public:
    void PrintContents()
    {
        std::cout << "(" << getName() << "," << getDiameter() << "," << getDirection() << ")";
    }
};

class fishObject : public MovingObject
{
public:
    void PrintContents()
    {
        std::cout << "(" << getName() << "," << getDiameter() << "," << getDirection() << ", [" << getDimension() << "], " << getMovingSpeed() << ")";
    }
};

I later set all these member variables inside a main function. The problem is when I try to output the contents of the member variables, all of them show up except for the string name.
Now, I’ve checked to make sure that the string gets set before calling the PrintContent() method, and it shows that the value is in the vector. However, when I debug through the code, the value is no longer there, instead containing an empty string.

Could someone with better c++ knowledge explain to me why this is happening? This is the main class:

int main()
{
        std::string type;
        Reader reader;

        while (!std::cin.eof())
        {
            try
            {
                std::string type;
                std::cin >> type;

                if (type =="int")
                {
                    reader.ReadDirection();
                }
                else if (type =="float")
                {
                    reader.ReadDiameter();
                }
                else if (type == "string")
                {
                    std::string name;
                    std::cin >> name;

                    if (hasDirection && hasDiameter)
                    {
                        int dimension;
                        if (diameter > 0 && diameter < 10)
                        {   
                            //fish
                            fishObject fish;
                            fish.setName(name);
                            fish.setDiameter(diameter);
                            fish.setDirection(direction);

                            dimension = fishDimension;
                            fishDimension += 50;
                            fish.setDimension(dimension);
                            fish.setMovingSpeed(0.1);
                            data.push_back(&fish);
                        }
                        else if (diameter >= 10 < 500)
                        {
                            //animal
                            animalObject animal;
                            animal.setName(name);
                            animal.setDiameter(diameter);
                            animal.setDirection(direction);

                            dimension = animalDimension;
                            animalDimension += 800;
                            animal.setDimension(dimension);
                            animal.setMovingSpeed(5.0); 
                            data.push_back(&animal);
                        }
                        else if (diameter >=500)
                        {
                            //star
                            starObject star;
                            star.setName(name);
                            star.setDiameter(diameter);
                            star.setDirection(direction);

                            dimension = starDimension;
                            starDimension += 5000;
                            star.setDimension(dimension);
                            star.setMovingSpeed(30.0);
                            data.push_back(&star);
                        }

                    }
                    else
                    {
                        throw (IncompleteData(name));
                    }
                }
            }
            catch (IncompleteData e)
            {
                std::cerr << "No diameter or direction given for object " << e.objectName << "\n";
            }
        }

Answer 1

The objects you push to the data vector are local because they are declared inside if/else blocks (see the declarations of fish and animal).

When you push the address of such an object to the vector, it will continue to point to the local object, which ceases to exist at the end of the local scope. You need to create objects that live beyond the local scope. One way of doing this is to create copies of the local objects on the heap and push those to the vector:

data.push_back(new fishObject(fish));

Of course this means that you get a memory leak unless you make sure you explicitly delete the elements of the vector some time before the end of the program. The usual recommendation to avoid having to think of this is to use a vector of std::unique_ptr<MovingObject> instead of a vector of naked pointers.