CODEWITHSUNDEEP
c++vector
user1232138
user1232138

Reputation: 5541

vector elements do not appear to be stored contigously

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';
}

Upvotes: 1

Views: 145

Answers (3)

Steve Jessop
Steve Jessop

Reputation: 279265

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.

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258618

A vector does store its elements in continuous memory. But the elements of vector<vector<int>> twoDv; are vectors, not ints, and the ints are stored in continuous memory internally, per vector.

enter image description here

Think of an array of pointers:

int* x[10];

The 10 pointers are stored in continuous memory, but what they point to doesn't have to be in continuous memory.

Upvotes: 5

Potatoswatter
Potatoswatter

Reputation: 137820

vector< vector< int > > is not a two-dimensional array like int[5][5]. It's an array of pointers to arrays. (To be more precise, it contains a sequence of std::vector objects containing pointers to integers.) Only the "rows" are contiguous. Different rows are not contiguous with each other because they may be stored in different malloc'ed blocks of memory.

Upvotes: 5

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