How can an object of a vector access the vector elements

Question

This is a program that simulates a simple resource gathering game The robots gather resources from the map and move around randomly each one doing some actions. My problem is that i want to access a vector from class map within the derivered class of robots "RescueBot" . The program is written in multiple files, header.h , header.cpp, main.cpp

I have a vector of objects type "Robots" and an example of my header.h file:

class Map{
  private:
    char World[20][20]; // The size of Map is 20 x 20
    vector<Robots*>RobotsVector; 
  public:
    Map();
    vector<Robots*>*getRobotsVector();

}

   // I access the vector with getRobotsVector() which belongs to Map class but returns 
   // Robot objects. Done this so i can access Robot objects within the Map class.

class Robots
{
private:
    //some variables

public:
    //some functions
    virtual void movement()=0; // this is the function that handles the robots moves
    virtual void action()=0; // this is the function that handles the robots actions
}

class RescueBot:public Robots{
   void movement();
   void action();
 //some unique RescueBot stuff here
}

This is from the header.cpp file:

#include "header.h"
vector<Robots*>*Map::getRobotsVector(){return &RobotsVector;}

 //example of object creation and pushing into vector
void Map::creation(){
   for (int x=0;x<4;x++){
    getRobotsVector()->push_back(new RescueBot);
   }
}

void RescueBot::action(){
    //do stuff

 for(int i=0;i<Map::getRobotsVector()->size();i++){

       //Here is the problem. I cant get inside the for loop

      Map::getRobotsVector()->at(i)->setDamaged(false); //Changes a flag in other objects

   }
}

I Have tried making the Robots class derivered class of Map. After that when i run it the vector i access in RescueBot::action is empty, while the actual vector has objects in it. If i dont make it derivered it doesnt compile.

How can i access the vector from within the RescueBot::action() ??

It cannot, unless you pass the vector to the constructor of the object, or set a pointer to the vector on each object that you add to that vector. Generally, neither one is a particularly good idea. — Sergey Kalinichenko, Dec 17 '13 at 19:46
Your thinking about it wrong. If RescueBot needs to modify Map, then it needs to take Map in as a parameter, either in the constructor or in the action command (personally, i'd favor the action command). — IdeaHat, Dec 17 '13 at 19:50

score 1 · Accepted Answer · answered Dec 17 '13 at 19:56

1

The problem is that you have no Map instances.

The way you're calling it currently would only work if the getRobotsVector method were static, but you don't want that.

The reason it works when you make the Robots class a derived class of Map is because Map::getRobotsVector() would just mean you want to invoke the getRobotsVector method on the instance that the RescueBot::action function is operating on.

The solution is to pass an instance of a Map to your action functions by reference.

This is what your action function would look like then:

void RescueBot::action(Map& map) {
    // Do whatever you want with the map here.
    // For example:
    map.getRobotsVector()->at(i)->setDamaged(false);
}

answered Dec 17 '13 at 19:56

user123

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Since the OP probably will have only 1 map active at a time, why wouldn't he want to make `getRobotsVector` `static`? It seems like a perfectly correct solution IMHO. – Paweł Stawarz Dec 17 '13 at 19:58
@PawełStawarz It's analogous to a singleton, which is a terrible thing to have for [some pretty good reasons](http://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons) – user123 Dec 17 '13 at 20:00
I dont have only 1 action function. I have 3 via polymorphism. When i change my rescuebot::action i get this error at the compiler : << prototype for 'void RescueBot::action(Map&)' does not match any in class 'RescueBot'|>> – DnkStyle Dec 18 '13 at 12:52
i tryed it the static way too but i get an error at header.cpp at this "vector*Map::getRobotsVector(){return &RobotsVector;}" The error is: undefined reference to `Map::RobotsVector' Do i need to change my getRobotsVector function to make it work ? – DnkStyle Dec 18 '13 at 14:18
All your `action` methods should take a `Map&` then. – user123 Dec 18 '13 at 14:36

How can an object of a vector access the vector elements

1 Answers1