boost::program_options的设置格式的实现

日前项目当中用到boost::program_options这个库，主要是用来parse commandline的，大致的功能如下：

以下代码摘自boost help

// Declare the supported options.
po::options_description desc("Allowed options");
desc.add_options()
    ("help", "produce help message")
    ("compression", po::value<int>(), "set compression level")
;

po::variables_map vm;
po::store(po::parse_command_line(ac, av, desc), vm);
po::notify(vm);

if (vm.count("help")) {
    cout << desc << "n";
    return 1;
}

if (vm.count("compression")) {
    cout << "Compression level was set to "
 << vm["compression"].as<int>() << ".n";
} else {
    cout << "Compression level was not set.n";
}

利用这个库，可以生成类似下列的help信息：

$ bin/gcc/debug/first --help
Allowed options:
  --help : produce help message
  --compression arg : set compression level

这边的设置功能很有意思，一条条直接插入，很简洁，如下：

desc.add_options()
    ("help", "produce help message")
    ("compression", po::value<int>(), "set compression level")
;

如何实现这样的功能的？其实在看了boost的源代码之后，就可以知道这个是利用重载()运算符实现的，下面，我简化了boost库的代码，写了一个测试的程序，如下。

#include <boost/shared_ptr.hpp>

#include <iostream>
#include <string>
#include <vector>
using namespace std;

namespace program_option_own
{
    class program_option_data
    {
    private:
        string _name;
        string _description;
    public:
        program_option_data( const char* name, const char * description):
            _name(name), _description(description)
        {
        };
        string& name(){return _name;};
        string& description(){return _description;};
    };

    class program_option_init
    {
    private:
        string _optname;
        std::vector < boost::shared_ptr<program_option_data> > m_options;
    public:
        program_option_init(const char* name):_optname(name){};
        program_option_init& add_option(){return *this;};
        program_option_init& add(boost::shared_ptr<program_option_data> desc){
            m_options.push_back(desc);
            return *this;
        };
        program_option_init& operator()(const char * name,
                                         const char * description){
            boost::shared_ptr<program_option_data> d(
                new program_option_data(name, description));
            add(d);
            return *this;
        };
        friend  ostream & operator << (ostream&, program_option_init &);
    };
     ostream & operator << (ostream & o, program_option_init & p)
    {
        o << p._optname << endl;
        for (int i = 0; i < p.m_options.size(); i++)
            o << p.m_options[i]->name() << " " << p.m_options[i]->description() << endl;
    }
}
int main( int argc, char ** argv )
{
    using namespace program_option_own;
    program_option_init set("title");
    set.add_option()
        ("--help", "show help")
        ("--c", "configuration")
        ;
    cout << set;

    return 0;
}

简化版当中，定义了2个类，program_option_data用来存储单个插入信息的string，program_option_init用来存储所有的program_option_data，并将它们保存在一个vector里面，用适当的方法打印出来。
下列是打印出来的值

[root@jsu27 program_option_own]# ./program_option_own.bin
title
--help show help
--c configuration

该方法设置简单明了，值得我们学习。