Iterator over a list of pairs in C++? - c++

Question

I defined a list of pairs and wish to access them using an iterator, following an example.
class A{
private:
list<pair<size_type,size_type> > l_edge_;
public:
void function() const{
list<pair<size_type,size_type> >::iterator Iter_;
Iter_ = l_edge_.begin();
}
}
However, I got a compilation error. How can I fix it?
error: no match for 'operator=' (operand types are 'const iterator
{aka const std::_List_iterator<std::pair<unsigned int, unsigned int> >}' and
'std::list<std::pair<unsigned int, unsigned int> >::const_iterator
{aka std::_List_const_iterator<std::pair<unsigned int, unsigned int> >}')

Answer 1

My guess is that you are trying to write a const member function, and not what you copied in the question:
void function() const
{
Iter_ = l_edge_.begin();
}
Now, since the function is const, the l_edge_ member is also const, and so, begin() returns a const_iterator instead of a plain iterator. But that hardly matters because the Iter_ member is also const, so it cannot be assigned to.
Usually you do not want to declare iterators as member variables, unless very special needs. Instead, just declare a local one when you need it, and of the appropriate constness:
class A
{
private:
list<pair<size_type,size_type> > l_edge_;
public:
//const member function
void function() const
{
list< pair<size_type,size_type> >::const_iterator iter = l_edge_.begin();
}
//non-const member function
void function()
{
list< pair<size_type,size_type> >::iterator iter = l_edge_.begin();
}
};