====== Passing Vectors of Object Pointers in C++ =======

Learning to use vectors in c++ is fairly straight forward. But passing the collection to a function and returning an instance is not demonstrated as often. 

The following code snippet should show a nice complete flow in one shot. Take a look:

<code cpp>

#include <iostream>
#include <vector>

using namespace std;

class Planet {
    public:
        int id;
        Planet(int num){
            id = num;
        }
};

class PlanetScanner {
    public:
        Planet* SelectPlanet(vector<Planet*> p){
            return p.at(1);
        }
};

int main(){

    Planet *mars = new Planet(1);
    Planet *earth = new Planet(2);
    Planet *venus = new Planet(3);

    vector<Planet*> planets;

    planets.push_back(mars);
    planets.push_back(earth);
    planets.push_back(venus);

    PlanetScanner *scanner = new PlanetScanner();
    
    Planet *selectedPlanet = scanner->SelectPlanet(planets);
    cout << "ID: " << selectedPlanet->id;

    return 1;
}

</code>

Everything above: the classes, the functions and everything is in one file. Structure really isn't the point here.

Start by including the ''vector'' library and ensuring the namespace of ''std'' is used. That way we can write ''vector'' instead of ''std::vector'', but either is fine.

Next, a simple ''Planet'' class that simply holds an ''id'' variable and can be set with the class constructor. The class itself isn't that important.

Going to skip the ''SelectPlanet'' function in the ''PlanetScanner'' class for a second and talk about the ''main'' function.

In ''main'', three ''Planet'' pointers are created with ''Planet'' instances.

Next, a vector containing ''Planet*'' pointers is created rather than a vector containing ''Planet'' instances.

Each of the three Planet pointers is pushed into the vector.

Now we can talk about the ''SelectPlanet'' function in the ''PlanetScanner'' class. I have created the class to handle the processing and returning of a planet simply to make the scope of returning an object safe.

The ''SelectPlanet'' function accepts a vector of ''Planet*'' pointers. It also returns a single ''Planet*'' pointer as opposed to returning a ''Planet'' instance.

The ''Planet'' pointer from index 1 of the vector is returned to the caller in ''main''.

A new instance of ''Planet*'' is declared and address of the returned pointer is used.

And finally to demonstrate use, the id variable from the planet instance is printed.