boost_1_45_0/libs/units/example/runtime_unit.cpp

// Boost.Units - A C++ library for zero-overhead dimensional analysis and 
// unit/quantity manipulation and conversion
//
// Copyright (C) 2003-2008 Matthias Christian Schabel
// Copyright (C) 2008 Steven Watanabe
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#include <map>
#include <iostream>
#include <boost/lexical_cast.hpp>
#include <boost/units/quantity.hpp>
#include <boost/units/cmath.hpp>
#include <boost/units/systems/si/length.hpp>
#include <boost/units/base_units/imperial/foot.hpp>

//[runtime_unit_snippet_1

namespace {

using namespace boost::units;
using imperial::foot_base_unit;

std::map<std::string, quantity<si::length> > known_units;

}

quantity<si::length> calculate(const quantity<si::length>& t) 
{
    return(boost::units::hypot(t, 2.0 * si::meters));
}

int main() 
{
    known_units["meter"] = 1.0 * si::meters;
    known_units["centimeter"] = .01 * si::meters;
    known_units["foot"] =
        conversion_factor(foot_base_unit::unit_type(), si::meter) * si::meter;
        
    std::string output_type("meter");
    std::string input;
    
    while((std::cout << "> ") && (std::cin >> input)) 
    {
        if(!input.empty() && input[0] == '#') 
        {
            std::getline(std::cin, input);
        }
        else if(input == "exit") 
        {
            break;
        }
        else if(input == "help") 
        {
            std::cout << "type \"exit\" to exit\n"
                "type \"return 'unit'\" to set the return units\n"
                "type \"'number' 'unit'\" to do a simple calculation"
                << std::endl;
        } 
        else if(input == "return") 
        {
            if(std::cin >> input) 
            {
                if(known_units.find(input) != known_units.end()) 
                {
                    output_type = input;
                    std::cout << "Done." << std::endl;
                } 
                else 
                {
                    std::cout << "Unknown unit \"" << input << "\""
                         << std::endl;
                }
            } 
            else 
            {
                break;
            }
        } 
        else 
        {
            try 
            {
                double value = boost::lexical_cast<double>(input);
                
                if(std::cin >> input) 
                {
                    if(known_units.find(input) != known_units.end()) 
                    {
                        std::cout << static_cast<double>(
                            calculate(value * known_units[input]) /
                            known_units[output_type])
                            << ' ' << output_type << std::endl;
                    } 
                    else 
                    {
                        std::cout << "Unknown unit \"" << input << "\""
                            << std::endl;
                    }
                } 
                else 
                {
                    break;
                }
            } 
            catch(...) 
            {
                std::cout << "Input error" << std::endl;
            }
        }
    }
}

//]