How to change the value of tuple in a list?

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C++

hello. I want to update values in tuples of a list. the tuples in the list contains vertices of a graph and the degree of each vertices. I used igraph library to work on the graph. I wrote the following code. but, the value of tuple in the list d is not changed (I bolded the line that I mean). how can I resolve the problem?

std::list<std::tuple<int, int> >::iterator it;
list<std::tuple<int, int> > d;

it = d.begin();

for (int i=0;i<igraph_vcount(&graph);i++)
{
    tuple<int, int> f1=   it._Ptr[0]._Myval;
    std::get<1>(f1) = static_cast<int>( VECTOR(v)[i]);
    std::advance(it,1);
    }

What I have tried:

I use visual studio 2012.