Peter M. Groen / cxxopts

Fixes #93. This adds C++17 only support for `std::optional` values for
command line parameters.

Fixes #96. Default values of options not specified on the command line
had a `count` of 1. It would be better if they had a count of 0 because
they were not actually specified, so that count is only for options
given by the user.

* Add default "default" value of "false" to boolean options, therefore allowing to call result["boolOpt"].as<bool>() without throwing an exception.

Fixes #83.

Adds an overload for positional arguments taking a
`std::initializer_list`. When using an `initializer_list` with one
argument, the function call is ambiguous matching both `std::string`
and `std::vector<std::string>`.

Fixes #84. Implicit values are not handled very well. For example:

    --foo false true
    --foo --bar

In the first, `false` is an argument to `--foo`, and then `true` is a
positional argument. In the second, because of the hyphen in `--bar`, `--foo`
is parsed with its implicit value. This seems inconsistent and unintuitive.

Better is that implicit values *never* consume the next parameter to be
completely consistent. This means that values with an implicit parameter
*must* be specified using the `--option=value` form.

The parsing for boolean options was broken by 6c9bae4a07 which added implicit
and default values, and the ability to parse boolean strings. Having an option
after the boolean tried to parse that into the boolean instead of as a
positional parameter.

See #84 for the bug report.

Fixes #54. Allow default and implicit values for booleans with multiple
boolean strings matched as values.

Fixes #74.

* Prevent malformed numbers from being parsed as correct numbers.

Fixes #78. If you passed a string for example "test" it would get parsed to 1400.
The problem was that the parser did not throw an exception when an incorrect char was encountered.
Also a number without 0x in front with hexadecimal digits in it got parsed.
The number was treated as a hexadecimal number but it was still calculated with base 10.
So now before the current char is used, it is checked if it is valid in the current base.
Furthermore the number 0x0 was not a valid number, it now is a special case in the `integer_pattern`.

* Fixed `integer_pattern` so it works correctly under clang. Added testcase for invalid integers and for 0x0 being a valid number.

This is far from ideal, but it's the first step in moving the parse
result to an immutable object so that the parser can be reused.

Adds generic parsing of types using `operator>>`, in particular allows
parsing of floats and doubles.

Fixes #39. Closes #40. This is an overhaul of the way that integer
arguments are parsed. Instead of using std::istream, which allows,
for example, negative integers for unsigned types, we use our own
parser.

This allows us to do proper range checking depending on the type,
and to correctly check for negative values passed to unsigned types.

This also allows the handling of base 16 numbers.

Fixes #57.

Allows spaces after the comma between the short and long option
specification.

Fixes #50. This enables -Wall -Wextra -Wshadow and sets -Werror so that
the build fails if there are any warnings. All warnings that came up are
also fixed

Fixes #44. Adds a helper function for checking required options.

Fixes #47. Adds support for options that only have a short option.

Fixes #36. This fixes handling arguments passed after `--` when no
positional arguments have been declared, or when the positional
arguments have all been used up. The bug was that the extra arguments
were lost. Now they are retained in the modified argv.

Fixes #33.

This PR uses cmake's interface library feature:

  An INTERFACE library target does not directly create build output,
  though it may have properties set on it and it may be installed,
  exported and imported.

This makes it easier to include the header only library in a cmake
project. After using add_subdirectory on the cxxopts directory, one
simply needs to include cxxopts in their target_link_libraries, which
will allow the user's target to inherit the properties of the cxxopts
header library (see changes to example and test).

Closes #32. This adds numbers as allowed values in all parts
of short and long option specifiers.

Fixes #30. The positional arguments are not overwritten when they
have already been given on the command line.