How to have an optional option value in boost program options?

Question

I am using the Boost program option and I want to offer an option which has three ways:

1. If not define
2. If defined but no value
3. If defined with a value

For example, I have a program that works on a file such as a.jpg, and I want to offer the user to be able to use it in the following scenarios:

myapp.exe a.jpg  : process jpeg 
myapp.exe a.jpg -e : process jpeg and generate report at the same directory as a.jpg
myapp.exe a.jpg -e c:\tmp\ : process jpeg and generate report at c:\tmp\

How can I do this with Boost program options?

Answer 1

You can achieve this effect by giving the value both a default_value and an implicit_value.

The default_value will be used when the option is not specified at all. The implicit_value will be used when the option is specific without a value. If a value is specified, it will override the default and implicit.

So, some code to do this could look something like this:

#include "boost/program_options.hpp"
#include <iostream>
#include <string>

using namespace std;

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

    po::options_description desc("Options");
    desc.add_options()
        ("process-jpeg,e", po::value<string>()->default_value("")->implicit_value("./"), "Processes a JPEG.");

    po::variables_map vm;
    try
    {
        po::store(po::parse_command_line(argc, argv, desc), vm);
        po::notify(vm);
    } catch (po::error& e) {
        cerr << "ERROR: " << e.what() << endl << endl << desc << endl;
        return 1;
    }

    string outputDir = vm["process-jpeg"].as<string>();
    if (outputDir.empty()) {
        cout << "-e was not provided on the command line" << endl;
    } else {
        cout << "-e is using directory: " << outputDir << endl;
    }
}

Running this example code prints:

$ ./jpg_processor
-e was not provided on the command line

$ ./jpg_processor -e
-e is using directory: ./

$ ./jpg_processor -e c:\tmp
-e is using directory: c:\tmp