I have the need to pass negative integer values to an option in an application, hence this change.

foo -bar -10

will set the bar option to negative ten.

foo -bar -10 -baz

will set the bar option to negative ten and create a -10 option that is set "-baz". It's a little weird...

This won't cause bugs with boolean options, since no value accepted as false starts with '-'.

the template specialization methods must be explicitly marked with inline, otherwise including flags.h twice will cause linker errors.

Error LNK2005 "class std::optional<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > __cdecl flags::detail::get<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >(class std::unordered_map<class std::basic_string_view<char,struct std::char_traits<char> >,class std::optional<class std::basic_string_view<char,struct std::char_traits<char> > >,struct std::hash<class std::basic_string_view<char,struct std::char_traits<char> > >,struct std::equal_to<class std::basic_string_view<char,struct std::char_traits<char> > >,class std::allocator<struct std::pair<class std::basic_string_view<char,struct std::char_traits<char> > const ,class std::optional<class std::basic_string_view<char,struct std::char_traits<char> > > > > > const &,class std::basic_string_view<char,struct std::char_traits<char> > const &)" (??$get@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@detail@flags@@YA?AV?$optional@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@std@@AEBV?$unordered_map@V?$basic_string_view@DU?$char_traits@D@std@@@std@@V?$optional@V?$basic_string_view@DU?$char_traits@D@std@@@std@@@2@U?$hash@V?$basic_string_view@DU?$char_traits@D@std@@@std@@@2@U?$equal_to@V?$basic_string_view@DU?$char_traits@D@std@@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string_view@DU?$char_traits@D@std@@@std@@V?$optional@V?$basic_string_view@DU?$char_traits@D@std@@@std@@@2@@std@@@2@@3@AEBV?$basic_string_view@DU?$char_traits@D@std@@@3@@Z) already defined in app.game-d.lib(appmain.obj) app.entry E:\repo\glmovement\.build\projects\app.game-d.lib(rendersystem.obj) 1

It is advantageous to package this project as a cmake-compatible module since we can then allow users to just add it to their existing build system. Note, this does not add cmake support for the test directory to keep the change set small.

The cmake build system provided here supports an installation via cmake or working as a submodule. The docs have been updated to reflect this.

Since only options can be retrieved by the template get<T> functions, positional arguments needs to be manually parsed which added unneeded verbosity to the code.

This pull request adds the following functions:

• std::optional<T> args::get<T>(size_t positional_index)
• T args::get<T>(size_t positional_index, , T&& default_value)

If the positional_index doesn't exist an std::nullopt or the default value is returned.

Also this pull request contains a fix for #8 and adds test cases for the aforementioned functions and fix.

Steps to reproduce:

1. Copy the example to a file named src.cpp

#include "flags.h"
#include <iostream>

int main(int argc, char** argv) {
  const flags::args args(argc, argv);

  const auto count = args.get<int>("count");
  if (!count) {
    std::cerr << "No count supplied. :(\n";
    return 1;
  }
  std::cout << "That's " << *count << " incredible, colossal credits!\n";

  if (args.get<bool>("laugh", false)) {
    std::cout << "Ha ha ha ha!\n";
  }
  return 0;
}

2. Add flags.h to the same folder.
3. Compile using clang src.cpp -std=c++17 -o program.exe
4. Run using .\program.exe --count 5 --laugh

Expected result:

That's 5 incredible, colossal credits!
Ha ha ha ha!

Actual result:

That's 5 incredible, colossal credits!

Environment:

• OS: Windows 10
• Compiler: Clang 9.0.0

Reason behind the issue:

template <>
std::optional<bool> get(const argument_map& options,
                        const std::string_view& option) {
  if (const auto value = get_value(options, option)) {
    return std::none_of(falsities.begin(), falsities.end(),
                        [&value](auto falsity) { return *value == falsity; });
  }
  return std::nullopt;
}

In the aforementioned code, the function cannot differentiate between the two scenarios:

1. .\program.exe --count 5 --laugh
2. .\program.exe --count 5

Since in both cases, args.get<bool>("laugh") will return std::nullopt.

Suggested fix: Check if the option exists in the options map.

template <>
std::optional<bool> get(const argument_map& options,
                        const std::string_view& option) {
  if (const auto value = get_value(options, option)) {
    return std::none_of(falsities.begin(), falsities.end(),
                        [&value](auto falsity) { return *value == falsity; });
  }
  if (options.find(option) != options.end())
      return true;
  return std::nullopt;
}

Out of laziness, and because std::experimental is such a long namespace, using std::experimental::{optional, nullopt, string_view} was used in the flags namespace. It should be removed when when both optional and string_view are moved into std proper.

1. Unit tests should be written using https://github.com/jimporter/mettle.
2. There should be a Travis CI build/test ticker in the README after tests are added.

This is sorts of a quick and dirty change, but it allows for multiple options to be set:

If this is passed on the command line:

-foo=x -foo=y -foo=z

Then args.values("foo") will return a vector with the values "x", "y", and "z".

All other APIs are preserved and unchanged.

Now that there are some basic unit tests, flag validation and multiple arguments should be added.

Currently thinking of a syntax similar to:

const flags::args args(argc, argv);
if (const auto opt = args.get_multiple<bool>("flag", "f", "no-flag", "other")) {
  const auto [argument, value] = *opt;
  // do something -- *argument will be the first of "flag", "f", or "no-flag" that matched
  // and *value will be its value.
}

for multiple arguments.