vector elements do not appear to be stored contigo

问题:

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';
}

回答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.

回答2:

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.

回答3:

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.

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.