Question

I have defined a vector which consist of pairs and vectors. For better readability, I do this

#include <iostream>
#include <vector>
using namespace std;
class foo {
public:
  vector< pair<int, vector<int> > > v;
  vector< pair<int, vector<int> > >::iterator it;
  void bar() 
  {
    for( it = v.begin(); it != v.end(); it++ ) {
      if ( 10 == (*it).first ) {
          vector<int> a = (*it).second;   // NOTE
          a[0] = a[0]*2;                  // NOTE
      }
    }
  }
};

int main()
{
  foo f;
  f.bar();
  return 0;
}

As you can see, I assign (*it).second to a variable a so that I manipulate easily. Problem is, when I change the value of a, the original vector v doesn’t change. I can resolve that changing a to (*it).second every where in the loop. However this will make the code hard for reading.

Is there any way to reflect the changes in a to the original v? I have to say call by reference like this

vector<int> a = &(*it).second;

doesn’t work

Answer 1

You need to declare a as a reference:

vector<int>& a = (*it).second;

Ampersand in std::vector<int> a = &(*it).second; is being parsed as adress-of operator and a isn’t a pointer – that’s why it doesn’t work/compile.