Add support for std::optional

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

.travis.yml

@@ -12,7 +12,8 @@ matrix:
           packages:
             - g++-4.9
           sources: &sources
-            - llvm-toolchain-precise-3.8
+            - llvm-toolchain-trusty-3.8
+            - llvm-toolchain-trusty-5.0
             - ubuntu-toolchain-r-test
     - os: linux
       env: COMPILER=g++-4.9 UNICODE_OPTIONS=-DCXXOPTS_USE_UNICODE_HELP=Yes
@@ -27,9 +28,7 @@ matrix:
         apt:
           packages:
             - g++-5
-          sources: &sources
-            - llvm-toolchain-precise-3.8
-            - ubuntu-toolchain-r-test
+          sources: *sources
     - os: linux
       env: COMPILER=g++-5 UNICODE_OPTIONS=-DCXXOPTS_USE_UNICODE_HELP=Yes
       addons:
@@ -53,9 +52,17 @@ matrix:
             - clang-3.8
             - libc++-dev
           sources: *sources
+    - os: linux
+      env: COMPILER=clang++-5.0 CMAKE_OPTIONS=-DCXXOPTS_CXX_STANDARD=17
+      addons:
+        apt:
+          packages:
+            - clang-5.0
+            - g++-5
+          sources: *sources
 script: >
     cmake -DCXXOPTS_BUILD_TESTS=ON -DCMAKE_CXX_COMPILER=$COMPILER
-    -DCMAKE_CXX_FLAGS=$CXXFLAGS $UNICODE_OPTIONS .
+    -DCMAKE_CXX_FLAGS=$CXXFLAGS $UNICODE_OPTIONS $CMAKE_OPTIONS .
     && make && make ARGS=--output-on-failure test
 
 before_install:

CMakeLists.txt

@@ -28,8 +28,14 @@ option(CXXOPTS_BUILD_EXAMPLES "Set to ON to build examples" ON)
 option(CXXOPTS_BUILD_TESTS "Set to ON to build tests" OFF)
 
 # request c++11 without gnu extension for the whole project and enable more warnings
-set(CMAKE_CXX_STANDARD   11)
+if (CXXOPTS_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD ${CXXOPTS_CXX_STANDARD})
+else()
+  set(CMAKE_CXX_STANDARD 11)
+endif()
+
 set(CMAKE_CXX_EXTENSIONS OFF)
+
 if(MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W2")
 elseif(CMAKE_CXX_COMPILER_ID MATCHES "[Cc]lang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")

include/cxxopts.hpp

@@ -38,6 +38,11 @@ THE SOFTWARE.
 #include <unordered_set>
 #include <vector>
 
+#ifdef __cpp_lib_optional
+#include <optional>
+#define CXXOPTS_HAS_OPTIONAL
+#endif
+
 namespace cxxopts
 {
   static constexpr struct {
@@ -697,6 +702,17 @@ namespace cxxopts
       value.push_back(v);
     }
 
+#ifdef CXXOPTS_HAS_OPTIONAL
+    template <typename T>
+    void
+    parse_value(const std::string& text, std::optional<T>& value)
+    {
+      T result;
+      parse_value(text, result);
+      value = std::move(result);
+    }
+#endif
+
     template <typename T>
     struct type_is_container
     {

test/options.cpp

@@ -454,3 +454,22 @@ TEST_CASE("Booleans", "[boolean]") {
 
   REQUIRE(result.count("others") == 1);
 }
+
+#ifdef CXXOPTS_HAS_OPTIONAL
+TEST_CASE("std::optional", "[optional]") {
+  std::optional<std::string> optional;
+  cxxopts::Options options("optional", " - tests optional");
+  options.add_options()
+    ("optional", "an optional option", cxxopts::value<std::optional<std::string>>(optional));
+
+  Argv av({"optional", "--optional", "foo"});
+
+  char** argv = av.argv();
+  auto argc = av.argc();
+
+  options.parse(argc, argv);
+
+  REQUIRE(optional.has_value());
+  CHECK(*optional == "foo");
+}
+#endif