Fix handling of implicit values

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.

CHANGELOG.md

@@ -3,6 +3,16 @@
 This is the changelog for `cxxopts`, a C++11 library for parsing command line
 options. The project adheres to semantic versioning.
 
+## 2.1
+
+### Changed
+
+* Options with implicit arguments now require the `--option=value` form if
+  they are to be specified with an option. This is to remove the ambiguity
+  when a positional argument could follow an option with an implicit value.
+  For example, `--foo value`, where `foo` has an implicit value, will be
+  parsed as `--foo=implicit` and a positional argument `value`.
+
 ## 2.0
 
 ### Changed

include/cxxopts.hpp

@@ -1522,7 +1522,7 @@ ParseResult::checked_parse_arg
   }
   else
   {
-    if (argv[current + 1][0] == '-' && value->value().has_implicit())
+    if (value->value().has_implicit())
     {
       parse_option(value, name, value->value().get_implicit_value());
     }
@@ -1696,7 +1696,7 @@ ParseResult::parse(int& argc, char**& argv)
         auto opt = iter->second;
 
         //equals provided for long option?
-        if (result[3].length() != 0)
+        if (result[2].length() != 0)
         {
           //parse the option given
 

test/options.cpp

@@ -218,7 +218,7 @@ TEST_CASE("Empty with implicit value", "[implicit]")
     ("implicit", "Has implicit", cxxopts::value<std::string>()
       ->implicit_value("foo"));
 
-  Argv av({"implicit", "--implicit", ""});
+  Argv av({"implicit", "--implicit="});
 
   char** argv = av.argv();
   auto argc = av.argc();
@@ -428,7 +428,7 @@ TEST_CASE("Booleans", "[boolean]") {
 
   options.parse_positional("others");
 
-  Argv av({"booleans", "--bool=false", "--debug", "true", "--timing", "extra"});
+  Argv av({"booleans", "--bool=false", "--debug=true", "--timing", "extra"});
 
   char** argv = av.argv();
   auto argc = av.argc();