The CmdLine Library#

Introduction#

This library assists with reading parameters from the command line. It supports different types of arguments, including floating point numbers, integers, and booleans. Moreover, it automatically generates a --help flag that provides custom information about the available flags.

Tutorial#

To use this library, you only need to familiarize yourself with a single class. This class is called CommandLineOptionParser. It’s in the namespace ae108::cmdline.

Example Application#

Let’s try using this class in a simple example. In this example, we are going greet the world if greetings are enabled via a command line flag.

First, we include the necessary headers and start with defining a main function.

// You should have received a copy of the GNU General Public License
// along with ae108. If not, see <https://www.gnu.org/licenses/>.

#include <ae108/cmdline/CommandLineOptionParser.h>
#include <iostream>

To parse the command line parameters, we’ll construct a CommandLineOptionParser by providing a stream. Let’s say we want to print error messages and warnings to stderr. In addition, we define a boolean variable enable_greeting that will store whether the greeting was enabled.

int main(const int argc, const char *const *const argv) {
  namespace cmdline = ae108::cmdline;

We’ll add a flag --enable_greeting (with a short form -g) together with a help text describing the flag. The CommandLineOptionParser class provides a withOption method that we are going to use to achieve that.

Now that we’ve configured the CommandLineOptionParser, the only thing that is missing is using it to parse the command line options in argc/argv.

  auto enable_greeting = false;
  cmdline::CommandLineOptionParser(std::cerr)

Finally we print a message if greetings have been enabled.

      .withOption("enable_greeting,g", "Print a greeting.", &enable_greeting)
      .parse(argc, argv);

  if (enable_greeting) {
    std::cout << "Hello world!" << '\n';
  }
}

Let’s see the application in action.

$ cmdline/examples/ae108-CmdLineExample
$ cmdline/examples/ae108-CmdLineExample --enable_greeting=yes
Hello world!
$ cmdline/examples/ae108-CmdLineExample --enable_greeting=no
$ cmdline/examples/ae108-CmdLineExample -g yes
Hello world!

As promised, there’s also a --help flag:

$ cmdline/examples/ae108-CmdLineExample --help
Command line options:
-h [ --help ]                Show this help.
-g [ --enable_greeting ] arg Print a greeting.

Moreover, a warning message is printed out to stderr if an unknown flag is used.

$ cmdline/examples/ae108-CmdLineExample --unknown_flag=123
Warning: The following options were not recognized by cmdline: '--unknown_flag=123'.

Most importantly, invalid command line parameters are rejected with a suitable error message.

$ cmdline/examples/ae108-CmdLineExample --enable_greeting=123
the argument ('123') for option '--enable_greeting' is invalid. Valid choices are 'on|off', 'yes|no', '1|0' and 'true|false'

Using the Example#

The full source code of the example is available in examples/Cmdline.cc. Here’s what it looks like:

 1// © 2020 ETH Zurich, Mechanics and Materials Lab
 2//
 3// This file is part of ae108.
 4//
 5// ae108 is free software: you can redistribute it and/or modify
 6// it under the terms of the GNU General Public License as published by
 7// the Free Software Foundation, either version 3 of the License, or any
 8// later version.
 9//
10// ae108 is distributed in the hope that it will be useful,
11// but WITHOUT ANY WARRANTY; without even the implied warranty of
12// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13// GNU General Public License for more details.
14//
15// You should have received a copy of the GNU General Public License
16// along with ae108. If not, see <https://www.gnu.org/licenses/>.
17
18#include <ae108/cmdline/CommandLineOptionParser.h>
19#include <iostream>
20
21int main(const int argc, const char *const *const argv) {
22  namespace cmdline = ae108::cmdline;
23
24  auto enable_greeting = false;
25  cmdline::CommandLineOptionParser(std::cerr)
26      .withOption("enable_greeting,g", "Print a greeting.", &enable_greeting)
27      .parse(argc, argv);
28
29  if (enable_greeting) {
30    std::cout << "Hello world!" << '\n';
31  }
32}

If you want to build it and try it out, then compile the executable target ae108-examples-Cmdline and run it with command line options of your choice.

Outlook#

We’ve seen many of the features in action, but there’s a bit more to explore. For instance, it’s possible to chain more than one call to withOption:

cmdline::CommandLineOptionParser(std::cerr)
    .withOption(/* ... */)
    .withOption(/* ... */)
    .parse(argc, argv);

Also, there is another overload of withOption that permits to add flags without a help text.

You can find additional information in the API documentation of the library.

In addition, the tests in cmdline/test/CommandLineOptionParser_Test.cc showcase the features in common use cases.

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