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Multiple sources

It is possible for program options to come from different sources. Here, the command line and a config file are used, and the values specified in both are combined, with preferrence given to the command line.

/* Shows how to use both command line and config file. */

#include <boost/program_options.hpp>
namespace po = boost::program_options;


#include <iostream>
#include <fstream>
using namespace std;


int main(int ac, const char* av[])
{
    try {
        po::options_description desc("Allowed options");
    
        // Declare a group of options that will be 
        // allowed only on command line
        po::options_description desc2("Generic options");
        desc2.add_options()
            ("version,v", "", "print version string")
            ("help", "", "produce help message")    
            ;
    
        // Declare a group of options that will be 
        // allowed both on command line and in
        // config file
        po::options_description desc3("Configuration");
        desc3.add_options()
            ("output,o", "file", "where to send output")
            ("magic", "value", "magic value for the program")
                .default_value("43")
            ;
        
        desc.add(desc2);    
        desc.add(desc3);

        // Parse command line and store results.
        po::variables_map vm;
        po::options_and_arguments oa1 = parse_command_line(ac, av, desc);
        po::store(oa1, vm, desc);
    
        // Parse config file and store results. Note
        // that different set of allowed options is used.
        po::variables_map cfg_file_vm;
        ifstream ifs("multiple_sources.cfg");
        po::options_and_arguments oa2 = parse_config_file(ifs, desc3);
        po::store(oa2, cfg_file_vm, desc3);
    
        // Chain the variable maps
        vm.next(&cfg_file_vm);
    
        if (vm.count("help")) {
            cout << desc << "\n";
            return 0;
        }

        if (vm.count("version")) {
            cout << "Multiple sources example, version 1.0\n";
            return 0;
        }
    
        if (!vm["output"].empty())
            cout << "output = " << vm["output"].as<string>() << "\n";
    
        // This would output the value of 'magic' given on command line
        // If there's no value on command line, the value from
        // config file will be used. If there's no value even
        // there, then default value, "43", given in option
        // declaration, will be used.
        cout << "magic = " << vm["magic"].as<int>() << "\n";    
    }
    catch(exception& e)
    {
        cout << e.what() << "\n";
        return 1;
    }    
    return 0;
}

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