boost::program_options for single-byte variables

Question

*emphasized text*How can I use Boost program options to accept single-byte variables from the command line?

Command line parameters of --id1=1 --id2=1 results in values of id1=49 (or '1', 0x31) and id2=1.

#include <stdint.h>
#include <iostream>
#include <boost/program_options.hpp>

using namespace std;

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

    const int myargc = 3;
    const char* myargv[] = {"foo","--id1=1","--id2=2" };

    uint8_t  id1;
    uint16_t id2; // works as expected.

    po::variables_map vm;
    po::options_description cmd_options( "Command options" );
    cmd_options.add_options()
    ( "id1", po::value<uint8_t >( &id1 )->default_value( 0 ), "A 1-byte ID" )
    ( "id2", po::value<uint16_t>( &id2 )->default_value( 0 ), "A 2-byte ID" )
    ;

    po::store( po::parse_command_line( myargc, myargv, cmd_options ), vm );
    po::notify( vm );
    // Using command line parameters of --id1=1 --id2=1,    
    // at this point, id1=49 (or '1', 0x31) and id2=1.
    cout << "BPO parsing of " << myargv[1] << " and " << myargv[2] << endl;
    cout << "id1: " <<      id1 << endl;
    cout << "id1: " << (int)id1 << endl;
    cout << "id2: " <<      id2 << endl;

    id1 = boost::lexical_cast<uint8_t>("1");
    id2 = boost::lexical_cast<int>("2");

    cout << "Using boost::lexical_cast" << endl;
    cout << "id1: " <<      id1 << endl;
    cout << "id1: " << (int)id1 << endl;
    cout << "id2: " <<      id2 << endl;

}

output is:

BPO parsing of --id1=1 and --id2=2
id1: 1
id1: 49
id2: 2
Using boost::lexical_cast
id1: 1
id1: 49
id2: 2

Boost eventually calls boost::lexical_cast("1")' which converts as a char rather than a numeric value - a "1" becomes a '1' which is 49.

Is there a way to change the boost::program_options::add_options() initialization to treat single-bye values as ints rather than string/char? If not, what options do I have to change the parsing or mapping? Obvious (but unfavorable) options are: [1] don't use char-like values [2] manually parse (bypass Boost) or [3] perform a secondary conversion after Boost does its parsing.

Answer 1

Create a numeric byte class that is the size of a byte but streams like a numeric value rather than streaming like a char.

#include <stdint.h>
#include <iostream>
#include <boost/program_options.hpp>

using namespace std;

struct NumByte
{
    uint8_t value;

    NumByte() : value() {}
    NumByte( const uint8_t &arg ) : value(arg) {}
    NumByte( const NumByte &arg ) : value(arg.value) {}

    operator uint8_t() const { return value; }

    friend istream& operator>>(istream& in, NumByte& valArg)
    {
        int i;
        in >> i;
        valArg.value = static_cast<uint8_t>(i);
        return in;
    }

    friend ostream& operator<<(ostream& out, NumByte& valArg)
    {
        out << static_cast<int>(valArg.value);
        return out;
    }
};

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

    const int myargc = 3;
    const char* myargv[] = {"foo","--id1=1","--id2=2" };

    NumByte  id1;
    uint16_t id2; 

    po::variables_map vm;
    po::options_description cmd_options( "Command options" );
    cmd_options.add_options()
      ( "id1", po::value<NumByte >( &id1 )->default_value( 0 ), "A 1-byte ID" )
      ( "id2", po::value<uint16_t>( &id2 )->default_value( 0 ), "A 2-byte ID" )
      ;

    po::store( po::parse_command_line( myargc, myargv, cmd_options ), vm );
    po::notify( vm );

    assert( sizeof(NumByte)==1 ); // insure the size of a numeric byte is the size of a byte.

    cout << "BPO parsing of " << myargv[1] << " and " << myargv[2] << endl;
    cout << "id1: " <<      id1 << endl;
    cout << "id1: " << (int)id1 << endl;
    cout << "id2: " <<      id2 << endl;

    id1 = boost::lexical_cast<NumByte>("1");
    id2 = boost::lexical_cast<int>("2");

    cout << "Using boost::lexical_cast" << endl;
    cout << "id1: " <<      id1 << endl;
    cout << "id1: " << (int)id1 << endl;
    cout << "id2: " <<      id2 << endl;

}

output is:

BPO parsing of --id1=1 and --id2=2
id1: 1
id1: 1
id2: 2
Using boost::lexical_cast
id1: 1
id1: 1
id2: 2

Answer 2

The behavior you observe has nothing to do with boost::program_options or boost::lexical_cast, it's just how stream extraction for uint8_t types works. The following demonstrates that

#include <iostream>
#include <sstream>

int
main()
{
   uint8_t id1;
   uint16_t id2;
   std::istringstream iss( "1 1" );
   iss >> id1 >> id2;
   std::cout << "id1 char:     " << id1 << std::endl;
   std::cout << "id1 uint8_t:  " << (uint8_t)id1 << std::endl;
   std::cout << "id1 int:      " << (int)id1 << std::endl;
   std::cout << "id2 uint16_t: " << (uint16_t)id2 << std::endl;
   std::cout << "id2 int:      " << (int)id2 << std::endl;
}

produces:

id1 char:     1
id1 uint8_t:  1
id1 int:      49
id2 uint16_t: 1
id2 int:      1

If you want to limit your program options to a single byte, I suggest using boost::numeric_cast

#include <stdint.h>
#include <iostream> 
#include <boost/bind.hpp>
#include <boost/numeric/conversion/cast.hpp>
#include <boost/program_options.hpp>

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

    const int myargc = 3;
    const char* myargv[] = {"foo","--id1=1","--id2=2" };

    unsigned  id1;
    uint16_t id2; // works as expected.

    po::options_description cmd_options( "Command options" );
    cmd_options.add_options()
        ( "id1", po::value<unsigned>( &id1 )->notifier([](unsigned value){ boost::numeric_cast<uint8_t>(value); } ), "A 1-byte ID" )
        ( "id2", po::value<uint16_t>( &id2 ), "A 2-byte ID" )
        ;

    po::variables_map vm;
    po::store( po::parse_command_line( myargc, myargv, cmd_options ), vm );
    po::notify( vm );
    std::cout << "id1: " << id1 << std::endl;
    std::cout << "id2: " << id2 << std::endl;
}

here is a sample session