如何返回在函数中构造的向量指针值

Question

I have a problem to get a return value of vector* value. I expect the return of value vector* have a components with size 1000 still after the get_vectorD exits. Let me know what the problem is in the following my code.

#include <iostream>
#include <vector>
using namespace std;

void get_vectorD(vector<int>* D) 
{
    vector<int>* d_vec = new vector<int>();
    for (int i = 0; i < 1000; i++) { d_vec->push_back(i); }
    D = d_vec; 
}

void main()
{   
    vector<int>* D = new vector<int>();
    get_vectorD(D);
    cout << D->at(0) << endl;
}

Answer 1

The problem is that you are passing the pointer by value. Pass it by a reference and it will work

void get_vectorD(vector<int>*& D)
{
    vector<int>* d_vec = new vector<int>;
    for (int i = 0; i < 1000; i++) { d_vec->push_back(i); }
    D = d_vec; 
}

and you can make use of function return value:

vector<int>* get_vectorD()
{
    vector<int>* d_vec = new vector<int>;
    for (int i = 0; i < 1000; i++) { d_vec->push_back(i); }
    return d_vec;
}

or use pointer of pointer

#include <iostream>
#include <vector>

using namespace std;

void get_vectorD(vector<int>** D)
{
    vector<int>* d_vec = new vector<int>;
    for (int i = 0; i < 1000; i++) { d_vec->push_back(i); }
    *D = d_vec;
}

int main()
{
    vector<int>* D;
    get_vectorD(&D);
    cout << D->at(0) << endl;
    delete D;
    return 0;
}

Answer 2

Following would work:

#include <iostream>
#include <vector>
using namespace std;

void get_vectorD(vector<int>*& D) 
{
    D = new vector<int>();
    for (int i = 0; i < 1000; i++) { D->push_back(i); } 
}

int main()
{   
    vector<int>* D;
    get_vectorD(D);
    cout << D->at(0) << endl;
}

Demo

Since you are passing the std::vector 's pointer by value, anything you do in the get_vectorD function won't have any effect on vector<int>* D . That's why you need to pass a reference to the pointer.

Also, there is no need to have a temporary d_vec variable in your get_vectorD function.

Better way

Furthermore, it would have the same effect and would be much less error prone not using the pointer at all like:

#include <iostream>
#include <vector>

void get_vectorD(std::vector<int>& D) {
    for (int i = 0; i < 1000; i++) {
        D.push_back(i);
    } 
}

int main() {
    std::vector<int> D;
    get_vectorD(D);
    std::cout << D.at(0) << std::endl;
}

Much better way to fill up the std::vector

Don't forget that you can fill up the std::vector by using std::iota like:

std::vector<int> D(1000);
std::iota(D.begin(), D.end(), 0);