Boost Program Options bool always True

Question

With program options, I am checking valid combinations of arguments. But for some reason, gpu argument is a bool and it is always true regardless if I set it to false on the command line. Is there a way that gpu option can be false if I specified it on the command line? I want to be able to make a bool variable that represents if the option on the command line was used.

Also I couldn't find any documentation on count() for variables_map. Is it a std::map function?

Partial Code:

namespace po = boost::program_options;
po::options_description desc("Allowed Options");
desc.add_options()
  ("help,h", "Produce help message")
  ("remove_database,r",po::value<std::vector<std::string>>
    (&remove_database),
    "Remove a pre-built database, provide a name(s) of the database")
  ("gpu,u", po::bool_switch()->default_value(false),
    "Use GPU? Only for specific algorithms");

po::variables_map vm;
po::store(po::parse_command_line(argc,argv,desc),vm);
po::notify(vm);

//Processing Cmd Args
bool help           = vm.count("help");
bool remove         = vm.count("remove_database");
bool gpu            = vm.count("gpu");

test(help,"help");
test(remove, "remove");
test(gpu, "gpu");

.....
void test(bool var1, std::string var2){
  if(var1)
    std::cout << var2 << " is active " << std::endl;
 else
    std::cout << var2 << " is not active " << std::endl;

Output:

$./a.out -r xx -u off
remove is active 
gpu is active
$./a.out -r xx -u false
remove is active 
gpu is active

`bool_switch` docs: "Works the same way as the 'value' function, but the created value_semantic won't accept any explicit value. So, if the option is present on the command line, the value will be 'true'." — chris, Aug 21 '15 at 22:43

score 7 · Answer 1 · edited May 23 '17 at 12:08

7

You're using a bool_switch. By default, the option will be false like you specified with ->default_value(false). Since it's a switch, the mere presence of -u or --gpu when you run the executable will turn the switch to true. It doesn't matter what you put after it.

See this answer for more usage details.

edited May 23 '17 at 12:08

Community

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answered Aug 21 '15 at 22:42

huu

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score 3 · Answer 2 · answered Jan 23 '17 at 13:27

It seems(*) count() is always 1 for a bool_switch. Hence one should not use:

bool help           = vm.count("help");

But instead use:

bool help           = vm["help"].as<bool>();

Or for "safety"(*):

bool help           = vm.count("help") ? vm["help"].as<bool>() : false;

(*) Delving into the documentation should tell exactly what is the exact and certain way to do things.

score 3 · Answer 3 · answered Nov 01 '17 at 21:44

Although not directly answer what OP has asked, I think this is an important note. According to the boost program_options spec spec, no matter what your default value is, when you specify the option from commandline it always turns the switch to true

So if you use default_value(true) for bool_switch(), you can't really turn it off...

Boost Program Options bool always True

3 Answers3

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