Question

Why does the last expected output differ from the actual output in the following code?

#include<iostream>
#include<fstream>
#include<istream>
#include<sstream>
#include<vector>

using namespace std;


int main()
{
        vector<int> v;
        for(int ii = 0; ii < 4; ii++){
            v.push_back(0);
        }

        vector<vector<int>> twoDv;

        for(int ii = 0; ii < 5; ii++){
            twoDv.push_back(v);
        }

        cout<<"Expected Output : " << &twoDv[0][0] <<'\t'<< (&twoDv[0][0] + 3) <<'\t'<< (&twoDv[0][3] + 1)<<'\n';
        cout<<"Actual Output   : " << &twoDv[0][0] <<'\t'<< &twoDv[0][3] <<'\t'<< &twoDv[1][0] << '\n';
}

Answer 1

The standard doesn’t say that &twoDv[1][0] is equal to &twoDv[0][3] + 1. It says that &twoDv[1] is equal to &twoDv[0] + 1, and that &twoDv[0][1] is equal to &twoDv[0][0] + 1.

Suppose for a moment that &twoDv[1][0] were equal to &twoDv[0][3] + 1, and then you did twoDv[0].resize(5);. Suddenly we have a conflict, &twoDv[0][3] + 1 can’t be the address of &twoDv[1][0] and also the address of &twoDv[0][4]. So the resize operation on twoDv[0] would have to invalidate iterators and references to the elements of another vector twoDv[1]. This would be very undesirable behavior.