how to return pair from a function?

I am trying to write a function which will return pair from function but I am getting error during compilation.

This is the whole file:

#include <iostream>
#include <map>
#include <utility>
using namespace std; 
typedef pair<const string, const double> pr;
typedef map<const string,pr > mpr;
mpr mymap;
pr getvalue(const string s)
{
    pr pValue;
    mpr::iterator iter = mymap.find(s);
    if(iter not_eq mymap.end())
    {
        pValue = (*iter).second;
    }
    return pValue;
}
int main( )
{
    getvalue("test");

}

Error Message:

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algobase.h:66, from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/char_traits.h:41, from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/ios:41, from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/ostream:40, from /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/iostream:40, from test8.cxx:1: /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_pair.h: In member function 'std::pair, std::allocator >, const double>& std::pair, std::allocator >, const double>::operator=(const std::pair, std::allocator >, const double>&)': /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_pair.h:68: error: non-static const member 'const std::basic_string, std::allocator > std::pair, std::allocator >, const double>::first', can't use default assignment operator /usr/ lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_pair.h:68: error: non-static const member 'const double std::pair, std::allocator >, const double>::second', can't use default assignment operator test8.cxx: In function 'pr getvalue(std::string)': test8.cxx:14: note: synthesized method 'std::pair, std::allocator >, const double>& std::pair, std::allocator >, const double>::operator=(const std::pair, std::allocator >, const double>&)' first required here

Please help me out.

---

The type pr is defined with both members of the pair being const . Once declared, the variable pValue cannot be changed in the assignment pValue = (*iter).second , because all it's members are effectively const.

The code can be modified to (which should compile);

pr getvalue(const string s)
{
    mpr::iterator iter = mymap.find(s);
    if(iter not_eq mymap.end())
    {
        return (*iter).second;
    }
    return pr();
}

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