How to make a loop that instanciates classes and sort them in a address list?

Question

Consider 2 variables (number of polygons and its coordinates) :

int numberPoly= 2;
float polycoord[18]= {0,50,0,0,0,0,50,0,0,0,50,0,50,50,0,50,0,0};

, a Model class (that is intended to store polygon classes to a list) :

class model{
    public:
        model();
        void affect(int id, int address){
            polyclasses[id]=address;
        }
    private:
        string name;
        vector<int> polyclasses;
};

, a Polygon class (that I have to sort in Model's polyclasses list) :

class polygone {
    public:
        polygone();
        void affect(int id, int coord){
            ABC[id]=coord;
        }
    private:
        int id;
        float ABC[9]={0.0};
};

I wanted to code a function (cf. "builder") that instanciate n Polygon classes and sort them (with their memory addresses like an id) in an array ("polyclasses" from Model class). So, I don't arrive. Here is a bit of my builder function not acomplished :

void builder(){
    int from = 0; int subfrom = 0;
    for(int i=0; i < numberPoly - 1; i++){
        from = 0; subfrom = 0;
        polygone poly();
        !!! need to put that instance in Model's polygon list !!!
        ...
        for(int j=from; j < (polycoord.size())-1; j++){
            poly.affect(subfrom, polycoord[j]) ...
            subfrom++;
        }
        from += 3;
    }
}

This is for my first c++ project. I'm coding a light 2d engine.

Clonk Clonk · Accepted Answer · 2019-09-12T08:22:23

You need to store pointer of instances in your vector and allocate your objects with new keyword. At destructor of your model yo uwill need to deletethe object to avoid a memory leak.

// Model.h
// Class name should begin with uppercase by convention
class Model{
    public:
        Model();
        ~Model();
        void builder(); 
        // Implementation should go in cpp file
        void affect(int id, int address);
    private:
        // Having a m_ prefix on private variable is useful to make your code more readable so a reader can easily know if a variable is private or not
        string m_name;
        vector<Polygon*> m_polyclasses;
};

// Polygone.h
class Polygone {
public:
    Polygone();
    // Don't forget destructor
    ~Polygone();
    // Implementation should go in cpp file
    void affect(int id, int address);
private:
    int m_id;
    // Use std::array in C++ and give meaningful name to your variable
    // float m_ABC[9]={0.0}; is replaced by :
    std::array<float, 9> m_coordinates;
};

// Model.cpp  
void Model::builder() {
    int from = 0; int subfrom = 0;
    for(int i=0; i < numberPoly - 1; i++){
        from = 0; subfrom = 0;
        Polygone * poly = new Polygone();
        // A pointer of poly is now stored in Model
        this->polyclasses.push_back(poly);
        // Your polygone object should initialized in the constructor or in a member function of the class Polygone.
        for(int j=from; j < (polycoord.size())-1; j++){
            poly->affect(subfrom, polycoord[j]) ...
            subfrom++;
        }
        from += 3;
    }
}

Model::~Model() {
    for(auto p: this->polyclasses) {
        // Avoid memory leak
        delete p; 
    }
    this->polyclasses.clear();
}

You can also store a std::unique_ptr instead of a plain pointer. In that case you don't need to delete.

How to make a loop that instanciates classes and sort them in a address list?

1 Answers