c++ - boost::program_options "polymorphic"参数

Question

我想使用 boost::program_options 创建一个可以按如下方式调用的可执行文件:

./example --nmax=0,10  # nmax is chosen randomly between 0 and 10
./example --nmax=9     # nmax is set to 9
./example              # nmax is set to the default value of 10

用最少的代码以类型安全的方式实现这一目标的最佳方法是什么？

Answer 1

I would like to use boost::program_options to create an executable which can be called as follows:

program_options 库非常灵活，可以通过使用流插入和提取运算符编写您自己的类轻松支持这一点。

#include <iostream>
#include <limits>
#include <stdlib.h>

#include <boost/lexical_cast.hpp>
#include <boost/program_options.hpp>


class Max
{
public:
    Max() :
        _max( std::numeric_limits<int>::max() )
    {

    }

    Max(
            int max
       ) :
        _max( max )
    {

    }

    Max(
            int low,
            int high
       )
    {
        int value = rand();
        value %= (high - low);
        value += low;
        _max = value;
    }

    int value() const { return _max; }

private:     
    int _max;
};

std::ostream&
operator<<(
        std::ostream& os,
        const Max& foo
        )
{
    os << foo.value();
    return os;
}

std::istream&
operator>>(
        std::istream& is,
        Max& foo
        )
{
    std::string line;
    std::getline( is, line );
    if ( !is ) return is;

    const std::string::size_type comma = line.find_first_of( ',' );
    try {
        if ( comma != std::string::npos ) {
            const int low = boost::lexical_cast<int>( line.substr(0, comma) );
            const int high = boost::lexical_cast<int>( line.substr(comma + 1) );
            foo = Max( low, high );
        } else {
            foo = Max( boost::lexical_cast<int>(line) );
        }
    } catch ( const boost::bad_lexical_cast& e ) {
        std::cerr << "garbage when convering Max value '" << line << "'" << std::endl;

        is.setstate( std::ios::failbit );
    }

    return is;
}

int
main( int argc, char** argv )
{
    namespace po = boost::program_options;

    Max nmax;

    po::options_description options;
    options.add_options()
        ("nmax", po::value(&nmax)->default_value(10), "random number range, or value" )
        ("help,h", po::bool_switch(), "help text")
        ;

    po::variables_map vm;
    try {
        po::command_line_parser cmd_line( argc, argv );
        cmd_line.options( options );
        po::store( cmd_line.run(), vm );
        po::notify( vm );
    } catch ( const boost::program_options::error& e ) {
        std::cerr << e.what() << std::endl;
        exit( EXIT_FAILURE );
    }

    if ( vm["help"].as<bool>() ) {
        std::cout << argv[0] << " [OPTIONS]" << std::endl;
        std::cout << std::endl;
        std::cout << "OPTIONS:" << std::endl;
        std::cout << options << std::endl;
        exit(EXIT_SUCCESS);
    }

    std::cout << "random value: " << nmax.value() << std::endl;
}

示例 session

samm:stackoverflow samm$ ./a.out
random value: 10
samm:stackoverflow samm$ ./a.out --nmax 55
random value: 55
samm:stackoverflow samm$ ./a.out --nmax 10,25
random value: 17
samm:stackoverflow samm$