What is wrong with this template code?

#include <iostream>
#include <map>
#include <vector>


template<typename T>
class foo {
    public:
        foo(T val) : m_Value(val) { };
        T get_value() const { return m_Value; };
        void set_value(const T& t) { m_Value=t; };
        bool operator<(const foo<T>& x) { return x.get_value() < m_Value; };
        bool operator==(const foo<T>& x) { return x.get_value() == m_Value; };
    private:
        T m_Value;
};

template<typename T>
class bar {
    public:
        bar() { };
        void print_first() const {
            typename s开发者_如何学运维td::map<foo<T>,std::vector<foo<T> > >::iterator it;
            it = m_Map.begin(); //ERROR!
            std::cout << it->first.get_value() << std::endl;
        };
    private:
        std::map<foo<T>,std::vector<foo<T> > > m_Map;
};

int main() {
    bar<int> b;
    b.print_first();
    return 0;
};

I am trying to write a container, but the member functions require use of an iterator, but when I try to actually use an iterator, I get an error:

testcase.cpp: In member function `void bar<T>::print_first() const [with T =
   int]':
testcase.cpp:33:   instantiated from here
testcase.cpp:24: error: no match for 'operator=' in 'it = std::map<_Key, _Tp,
   _Compare, _Alloc>::begin() const [with _Key = foo<int>, _Tp =
   std::vector<foo<int>, std::allocator<foo<int> > >, _Compare =
   std::less<foo<int> >, _Alloc = std::allocator<std::pair<const foo<int>,
   std::vector<foo<int>, std::allocator<foo<int> > > > >]()'
/usr/include/c++/3.3.3/bits/stl_tree.h:184: error: candidates are:
   std::_Rb_tree_iterator<std::pair<const foo<int>, std::vector<foo<int>,
   std::allocator<foo<int> > > >, std::pair<const foo<int>,
   std::vector<foo<int>, std::allocator<foo<int> > > >&, std::pair<const
   foo<int>, std::vector<foo<int>, std::allocator<foo<int> > > >*>&
   std::_Rb_tree_iterator<std::pair<const foo<int>, std::vector<foo<int>,
   std::allocator<foo<int> > > >, std::pair<const foo<int>,
   std::vector<foo<int>, std::allocator<foo<int> > > >&, std::pair<const
   foo<int>, std::vector<foo<int>, std::allocator<foo<int> > >
   >*>::operator=(const std::_Rb_tree_iterator<std::pair<const foo<int>,
   std::vector<foo<int>, std::allocator<foo<int> > > >, std::pair<const
   foo<int>, std::vector<foo<int>, std::allocator<foo<int> > > >&,
   std::pair<const foo<int>, std::vector<foo<int>, std::allocator<foo<int> > >
   >*>&)

What is it that i'm doing wrong? Thanks in advance.

print_first is a const method. Therefore the member m_Map is also const, and its begin method doesn't return an ordinary iterator, but a const_iterator. Change

typename std::map<foo<T>,std::vector<foo<T> > >::iterator it;

to

typename std::map<foo<T>,std::vector<foo<T> > >::const_iterator it;

and you should be good to go.