Sort an incoming map by key length, ties broken by string sorted order

sorted_map.cpp

/*********************************************************************
 *  Compilation:  g++ -std=c++11 map.cpp -o map.exe
 *  Execution:    map.exe < input.txt
 *  
 *  Output the contents of a sorted map
 *  
 *  This is a toy code snippet for using C++11 types to handle
 *  an input stream, store the incoming data columns by line, 
 *  and sort the output list according to length of key,
 *  with ties broken by string sort order (map default).
 *  
 *  $ cat input.txt
 *    Jane 20
 *    John 40
 *    Joe 30
 *    Frank 50
 *
 *  $ map.exe < input.txt
 *    Input Map ...
 *    <name: Joe, age: 30>
 *    <name: Jane, age: 20>
 *    <name: John, age: 40>
 *    <name: Frank, age: 50>
 *********************************************************************/
 
#include <iostream> // std::cout, std::cin
#include <string>   // std::string
#include <map>      // std::map

struct cmp_by_length 
{
    bool operator( )(const std::string& a, const std::string& b) const 
    {
        if (a.length() < b.length())
            return true;
        else if (a.length() == b.length())
            return a < b;
        else false;
    }
};

int main( )
{
    std::map<std::string, int, cmp_by_length> m;

    std::string key;
    int value;
    
    while (std::cin >> key >> value)
    {
        m[key] = value;
    }
    
    std::cout << "Input Map ..." << std::endl;
    for (auto itr = m.begin(), end = m.end(); itr != end; ++itr)
    {
        std::cout << "<name: " << itr->first;
        std::cout << ", age: " << itr->second;
        std::cout << ">" << std::endl;
    }
    
    return 0;
}