C++ how to modify a 2D vector of objects size in a function initialized in the constructor?

Question

Since I'm Italian, the function names are in Italian but really simple to understand: immagine=image, inserisci=insert, altezza=height, larghezza=width,mosaico =mosaic, righe=rows, colonne=columns.

So this program has 2 classes: an image with its attributes and a mosaic, which contains n images and this is represented through a 2D vector of obj Image (Immagine). The 2D vector has to be initialized in the constructor with r rows and c columns and using the inserisci (insert/add) function will then grow its dimension. Then if the element passed in the insert function has more rows/columns the insert function has to add the needed rows and columns in order to insert the element.

The problem is that even though I used pointers/references, every time I try to insert an element with a size that is bigger than the one initialized in the constructor, it gives me an error, meaning that the 2D vector that I modify in the insert function is not really edited... (look at the main when I insert: m2.inserisci(i4, 4, 4, &m2.immagini) since 4 rows > inital size of rows and same for columns gives me a runnning error..) Hope it is clear. This is my code:

#include <iostream>
#include <string>
#include <vector>
#include <sstream>

using namespace std;

class Immagine{
private:
        string nome;

public:
        int altezza;
        int larghezza;
        Immagine(string,int,int);
        string toString();
};
Immagine::Immagine(string n, int a, int l){
    nome = n;
    altezza = a;
    larghezza= l;
}
string Immagine::toString(){ 
    return nome;
        }

class Mosaico{
public:
    Mosaico(int, int,Immagine,vector< vector<Immagine> >*);

    int getRighe();
    int getColonne();
    string getImmagine(int,int);
    bool exist(int, int);
    Immagine getIm(int,int);
    void inserisci(Immagine,int,int,vector< vector<Immagine> >*);
    vector< vector<Immagine> > immagini;
    vector< vector<Immagine> >* aPointer= &immagini;
};
    Mosaico::Mosaico(int r, int c,Immagine imm, vector< vector<Immagine> >* immag ){
    (*immag).resize(r);
    for(int i=0; i<(*immag).size(); i++)
        for(int j=0; j<c; j++)
            (*immag)[i].insert((*immag)[i].begin()+j,imm);
}   
bool Mosaico::exist(int r, int c){
    for(int i = 0; i < getRighe(); i++){
        for(int j=0; j<getColonne(); j++){
            if(i==r && j==c && immagini[r][c].toString()!= " "){
                return true;
            }
        }
    }     
    return false;
}
int Mosaico::getRighe(){
    return immagini.size();
    }
int Mosaico::getColonne(){
    return immagini[1].size();
    }
string Mosaico::getImmagine(int r, int c){
    if(exist(r,c))
    return immagini[r][c].toString();
    }
Immagine Mosaico::getIm(int r, int c){
    return immagini[r][c];
}


void Mosaico::inserisci(Immagine imm,int r, int c, vector< vector<Immagine> >* immag){
        if(r<(*immag).size() && c<(*immag)[0].size()){
        (*immag)[r][c]=imm;
        }
        else if(r>=(*immag).size() && c>=(*immag)[0].size()){
            (*immag).resize(r);
            for(int i=0; i<r; i++){
                for(int j=(*immag)[0].size(); j<c; j++){
                    (*immag)[i].insert((*immag)[i].begin()+j, imm);
                }
            }
            (*immag)[r][c]=imm;
            }
        else if(r>=(*immag).size() && c<(*immag)[0].size()){
            (*immag).resize(r);
            (*immag)[r][c]=imm;
        }
    else if(r<(*immag).size() && c>=(*immag)[0].size()){
            for(int i=0; i<(*immag).size(); i++){
                for(int j=(*immag)[0].size(); j<c; j++){
                    (*immag)[i].insert((*immag)[i].begin()+j, imm);
                }
            }
            (*immag)[r][c]=imm;
            }

}

int main() {
    Immagine i1 ("I01",300,200);
    Immagine i2 ("I02",300,400);
    Immagine i3 ("I03",400,200);
    Immagine i4 ("I04",400,400);
    cout << "Creo un mosaico 2x2 con quattro immagini" <<endl;
    Mosaico m2(2,2,i1,&m2.immagini) ;
    cout<<m2.getRighe()<<endl;
    cout<<m2.getColonne()<<endl;
    for (int i=0; i < m2.getRighe(); i++) {
        for (int j=0; j < m2.getColonne(); j++){
            //if(m1.exist(i,j))
            cout<<m2.getImmagine(i,j);
        }
        cout<<endl;
    }
    m2.inserisci(i1, 0, 0, &m2.immagini);
    m2.inserisci(i2, 0, 1, &m2.immagini);
    m2.inserisci(i3, 1, 0, &m2.immagini);
    m2.inserisci(i4, 4, 4, &m2.immagini); //HERE IS WHERE I GET THE ERROR
    cout <<"Stampa mosaico: "<<endl;
    for (int i=0; i < m2.getRighe(); i++) {
        for (int j=0; j < m2.getColonne(); j++){
            cout<<m2.getImmagine(i,j);
        }
        cout<<endl;
    }
}

By the way, this was a Java exercise and I tried to do it in C++. Thanks in advance!

Answer 1

Before I'll get to the details of the issues, let me first state that there are a lot of things in the code I'd do differently. Most prominently, I'd probably use a flat vector<Image> for storage instead of a vector<vector<..>> . But such things are better suited for CodeReview in my opinion (once the code is working).

---

The biggest problems in the OP's code are within the implementation of the inserisci function:

 void Mosaico::inserisci(Immagine imm,int r, int c, vector< vector<Immagine> >* immag) { if(r < (*immag).size() && c < (*immag)[0].size()){ (*immag)[r][c]=imm; }

This part is fine, if we assume that (*immag)[i].size() is the same for all i in [0, (*immag).size()) . Let us call this assumption A. The [x, y) notation stands for a half-open-on-right interval (x is within that interval, y is not). Note that you can replace (*e).m with e->m , which I'll do in the following.

The assumption A is a class invariant: after every member function (except for the destructor), this class invariant must hold. Continuing with the inserisci function:

 else if(r>=(*immag).size() && c>=(*immag)[0].size()){ (*immag).resize(r);

The *immag vector now has r elements. This is insufficient if you want to access the r th element, since indexing starts with 0. You need to have at least r+1 elements. Replace the above resize with:

        int newRowCount = r+1;
        (*immag).resize(newRowCount);

Continuing with the OP's function:

 for(int i=0; i<r; i++){

This has the same off-by-one bug: replace r with newRowCount , or simply immag->size() .

         for(int i=0; i<immag->size(); i++){

Continuing with the OP's function:

 for(int j=(*immag)[0].size(); j<c; j++){ (*immag)[i].insert((*immag)[i].begin()+j, imm); } }

With the above resize , we have added newRowCount - immag->size() new elements to the immag vector. Those new elements are vectors of the size 0. To be able to access the c th element of such a vector, we need to add at least c+1 elements to it.

The value of (*immag)[0].size() however changes after the first iteration of the outer loop such that (*immag)[0].size() == c for the remaining (*immag)[i] elements where i is in [1, newRowCount) . This code won't add any new columns to those rows. A simple fix is:

            int newColumnCount = c+1;
            for(int j=(*immag)[i].size(); j < newColumnCount; j++){
                (*immag)[i].insert((*immag)[i].begin()+j, imm);
            }
        }

 (*immag)[r][c]=imm; }

A note related to code review: You can very easily resize the inner elements as well, using the resize-overload which takes an additional argument:

(*immag)[i].resize(newColumnCount, imm);

Continuing with the OP:

 else if(r>=(*immag).size() && c<(*immag)[0].size()){ (*immag).resize(r);

This suffers from the same off-by-one bug again. You need to have at least r+1 elements to access the r th element in the following

 (*immag)[r][c]=imm; }

And in the following piece of code, the same issues appear as in the second branch (that is, (*immag)[0].size() instead of referring to the current element's size, and j < c instead of j < newColumnCount ).

 else if(r<(*immag).size() && c>=(*immag)[0].size()){ for(int i=0; i<(*immag).size(); i++){ for(int j=(*immag)[0].size(); j<c; j++){ (*immag)[i].insert((*immag)[i].begin()+j, imm); } } (*immag)[r][c]=imm; } }