C++, I want to know how to allocate memory vector< pair<int, int> > *

Question

I have an error _Has_unused_capacity() vector... I don't know how to allocate dynamic memory

I try to push_back, but the error occurred

vector< pair<int, int> > * v;

void Some_Function(){
   int m=3;
   int idx1=1;
   int idx2=2;
   int idx3=3;

   for(int i = 0; i<m; i++) {
       v[idx1].push_back(make_pair(idx2, idx3));      
       v[idx2].push_back(make_pair(idx1, idx3));
   }
}

Answer 1

You have a poiter to a vector which points to nowhere. Either allocate memory for your vector ( not recommended though) or don't use a pointer.

vector< pair<int, int> > * v = new vector<pair<int, int>>[2];
V[idx].push_back(make_pair(idx2, idx3));

Don't forget to delete your vector when you are done with that.

delete [] v;

The better way is using smart pointers, here is an example with shared_ptr:

#include <iostream>

#include <vector>
#include <memory>

using namespace std;
using vecPair = vector<pair<int, int>>;

// deallocator for an array of vectors
void deleter(vecPair* x)
{
    delete[] x;
}

int main() {
    shared_ptr<vecPair> v;
    v.reset(new vecPair[2], deleter);
    int a = 1;
    int b = 2;
    int c = 3;
    int d = 4;
    v.get()[0].push_back(make_pair(a, b));
    v.get()[1].push_back(make_pair(c, d));
}