Feature add variant_cast (#129)

* Added global "variant_cast" method Added 4 overloads of the method "variant_cast" In order to move values out, to return values by reference or to return a pointer to the underlying values, when the type does match: template<class T> T variant_cast(variant&& operand); template<class T> T variant_cast(variant& operand); template<class T> T variant_cast(const variant& operand); template<class T> const T* variant_cast(const variant* operand) RTTR_NOEXCEPT; template<class T> T* variant_cast(variant* operand) RTTR_NOEXCEPT; Additionally: - added non-const version of variant::get_value() - fixed copy-right year in root CMakeLists.txt file Fixes #108
rttrorg · Mar 8, 2018 · 6e28c69 · 6e28c69
1 parent 0b4d72f
commit 6e28c69
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Showing 8 changed files with 511 additions and 7 deletions.
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -1,6 +1,6 @@
 ####################################################################################
 #                                                                                  #
-#  Copyright (c) 2014, 2015 - 2017 Axel Menzel <info@rttr.org>                     #
+#  Copyright (c) 2014 - 2018 Axel Menzel <info@rttr.org>                           #
 #                                                                                  #
 #  This file is part of RTTR (Run Time Type Reflection)                            #
 #  License: MIT License                                                            #

diff --git a/src/rttr/detail/misc/std_type_traits.h b/src/rttr/detail/misc/std_type_traits.h
@@ -248,6 +248,9 @@ using decay_t = typename std::decay<T>::type;
 template<typename T>
 using add_const_t = typename std::add_const<T>::type;
 
+template<typename T>
+using add_lvalue_reference_t = typename std::add_lvalue_reference<T>::type;
+
 /////////////////////////////////////////////////////////////////////////////////////
 
 } // end namespace detail

diff --git a/src/rttr/detail/variant/variant_data_policy.h b/src/rttr/detail/variant/variant_data_policy.h
@@ -652,7 +652,7 @@ struct RTTR_API variant_data_policy_empty
             }
             case variant_policy_operation::GET_VALUE:
             {
-                arg.get_value<void*>() = nullptr;
+                arg.get_value<const void*>() = nullptr;
                 break;
             }
             case variant_policy_operation::GET_TYPE:
@@ -767,7 +767,7 @@ struct RTTR_API variant_data_policy_void
             }
             case variant_policy_operation::GET_VALUE:
             {
-                arg.get_value<void*>() = nullptr;
+                arg.get_value<const void*>() = nullptr;
                 break;
             }
             case variant_policy_operation::GET_TYPE:

diff --git a/src/rttr/detail/variant/variant_impl.h b/src/rttr/detail/variant/variant_impl.h
@@ -40,6 +40,11 @@
 
 namespace rttr
 {
+namespace detail
+{
+template<typename T>
+using variant_t = remove_cv_t<remove_reference_t<T>>;
+}
 
 /////////////////////////////////////////////////////////////////////////////////////////
 
@@ -130,13 +135,24 @@ RTTR_INLINE bool variant::operator>(const variant& other) const
 
 /////////////////////////////////////////////////////////////////////////////////////////
 
+template<typename T>
+RTTR_INLINE T& variant::get_value()
+{
+    using namespace detail;
+    auto result = unsafe_variant_cast<variant_t<T>>(this);
+
+    return *result;
+}
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
 template<typename T>
 RTTR_INLINE const T& variant::get_value() const
 {
-    const void* value;
-    m_policy(detail::variant_policy_operation::GET_VALUE, m_data, value);
-    using nonRef = detail::remove_cv_t<T>;
-    return *reinterpret_cast<const nonRef*>(value);
+    using namespace detail;
+    auto result = unsafe_variant_cast<variant_t<T>>(this);
+
+    return *result;
 }
 
 /////////////////////////////////////////////////////////////////////////////////////////
@@ -390,6 +406,93 @@ RTTR_INLINE T variant::convert(bool* ok) const
     return convert_impl<T>(ok);
 }
 
+/////////////////////////////////////////////////////////////////////////////////////////
+/////////////////////////////////////////////////////////////////////////////////////////
+/////////////////////////////////////////////////////////////////////////////////////////
+
+namespace detail
+{
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE T* unsafe_variant_cast(variant* operand) RTTR_NOEXCEPT
+{
+    const void* value;
+    operand->m_policy(detail::variant_policy_operation::GET_VALUE, operand->m_data, value);
+    return reinterpret_cast<T*>(const_cast<void*>(value));
+}
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE const T* unsafe_variant_cast(const variant* operand) RTTR_NOEXCEPT
+{
+    return unsafe_variant_cast<const T>(const_cast<variant*>(operand));
+}
+
+} // end namespace detail
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE T variant_cast(const variant& operand)
+{
+    using namespace detail;
+    static_assert(std::is_constructible<T, const variant_t<T>&>::value,
+                  "variant_cast<T>(variant&) requires T to be constructible from const remove_cv_t<remove_reference_t<T>>&");
+
+    auto result = unsafe_variant_cast<variant_t<T>>(&operand);
+
+    using ref_type = conditional_t<std::is_reference<T>::value, T, add_lvalue_reference_t<T>>;
+    return static_cast<ref_type>(*result);
+}
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE T variant_cast(variant& operand)
+{
+    using namespace detail;
+    static_assert(std::is_constructible<T, variant_t<T>&>::value,
+                  "variant_cast<T>(variant&) requires T to be constructible from remove_cv_t<remove_reference_t<T>>&");
+
+    auto result = unsafe_variant_cast<variant_t<T>>(&operand);
+
+    using ref_type = conditional_t<std::is_reference<T>::value, T, add_lvalue_reference_t<T>>;
+    return static_cast<ref_type>(*result);
+}
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE T variant_cast(variant&& operand)
+{
+    using namespace detail;
+    static_assert(std::is_constructible<T, variant_t<T>>::value,
+                  "variant_cast<T>(variant&&) requires T to be constructible from remove_cv_t<remove_reference_t<T>>");
+    auto result = unsafe_variant_cast<variant_t<T>>(&operand);
+    return std::move(*result);
+}
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE T* variant_cast(variant* operand) RTTR_NOEXCEPT
+{
+    using namespace detail;
+    return (type::get<T>() == operand->get_type()) ?
+            unsafe_variant_cast<T>(operand) : nullptr;
+}
+
+/////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+RTTR_INLINE const T* variant_cast(const variant* operand) RTTR_NOEXCEPT
+{
+    return variant_cast<T>(const_cast<variant*>(operand));
+}
+
 /////////////////////////////////////////////////////////////////////////////////////////
 
 } // end namespace rttr

diff --git a/src/rttr/variant.h b/src/rttr/variant.h
@@ -52,6 +52,11 @@ class instance;
 
 namespace detail
 {
+    template<class T>
+    RTTR_INLINE T* unsafe_variant_cast(variant* operand) RTTR_NOEXCEPT;
+    template<class T>
+    RTTR_INLINE const T* unsafe_variant_cast(const variant* operand) RTTR_NOEXCEPT;
+
     struct data_address_container;
     template<typename T>
     struct empty_type_converter;
@@ -423,6 +428,32 @@ class RTTR_API variant
          */
         bool is_sequential_container() const;
 
+        /*!
+         * \brief Returns a reference to the containing value as type \p T.
+         *
+         * \code{.cpp}
+         *  struct custom_type
+         *  {
+         *     //...
+         *  };
+         *
+         *  variant var = custom_type{};
+         *  if (var.is_type<custom_type>())                         // yields to true
+         *      custom_type& value = var.get_value<custom_type>();  // extracts the value by reference
+         * \endcode
+         *
+         * \remark Only call this method when it is possible to return the containing value as the given type \p T.
+         *         Use therefore the method \ref is_type().
+         *         Otherwise the call leads to undefined behaviour.
+         *         Also make sure you don't clean this variant, when you still hold a reference to the containing value.
+         *
+         * \see is_type()
+         *
+         * \return A reference to the stored value.
+         */
+        template<typename T>
+        T& get_value();
+
         /*!
          * \brief Returns a reference to the containing value as type \p T.
          *
@@ -1067,13 +1098,107 @@ class RTTR_API variant
         friend struct detail::variant_data_base_policy;
         friend struct detail::variant_data_policy_nullptr_t;
         friend RTTR_API bool detail::variant_compare_less(const variant&, const type&, const variant&, const type&, bool& ok);
+        template<class T>
+        friend RTTR_INLINE T* detail::unsafe_variant_cast(variant* operand) RTTR_NOEXCEPT;
+
 
         detail::variant_data            m_data;
         detail::variant_policy_func     m_policy;
 };
 
 /////////////////////////////////////////////////////////////////////////////////////////
 
+/*!
+ * \brief Returns a reference to the containing value as type \p T.
+ *
+ * \code{.cpp}
+ *
+ *  variant var = std::string("hello world");
+ *  std:string& value_ref = variant_cast<std::string&>(var);  // extracts the value by reference
+ *  std:string value = variant_cast<std::string>(var);        // copies the value
+ *
+ * \endcode
+ *
+ * \remark Extracting a value type, which is not stored in the variant, leads to undefined behaviour.
+ *         No exception or error code will be returned!
+ */
+template<class T>
+T variant_cast(const variant& operand);
+
+/*!
+ * \brief Returns a reference to the containing value as type \p T.
+ *
+ * \code{.cpp}
+ *
+ *  variant var = std::string("hello world");
+ *  std:string& value_ref = variant_cast<std::string&>(var);  // extracts the value by reference
+ *  std:string value = variant_cast<std::string>(var);        // copies the value
+ *
+ * \endcode
+ *
+ * \remark Extracting a value type, which is not stored in the variant, leads to undefined behaviour.
+ *         No exception or error code will be returned!
+ */
+template<class T>
+T variant_cast(variant& operand);
+
+/*!
+ * \brief Move the containing value from the variant into a type \p T.
+ *
+ * \code{.cpp}
+ *
+ *  variant var = std::string("hello world");
+ *  std::string& a = variant_cast<std::string&>(var);
+ *  std:string b = variant_cast<std::string>(std::move(var)); // move the value to 'b'
+ *  std::cout << "a: " << a << std::endl; // is now empty (nothing to print)
+ *  std::cout << "b: " << b << std::endl; // prints "hello world"
+ *
+ * \endcode
+ *
+ * \remark Extracting a value type, which is not stored in the variant, leads to undefined behaviour.
+ *         No exception or error code will be returned!
+ */
+template<class T>
+T variant_cast(variant&& operand);
+
+/*!
+ * \brief Returns a pointer to the containing value with type \p T.
+ *        When the containing value is of type \p T, a valid pointer to the type will be returned.
+ *        Otherwise a `nullptr` is returned.
+ *
+ * \code{.cpp}
+ *
+ *  variant var = std::string("hello world");
+ *  std:string* a = variant_cast<std::string>(&var);  // performs an internal type check and returns the value by reference
+ *  int* b        = variant_cast<int>(&var);
+ *  std::cout << "a valid: " << a != nullptr << std::endl;  // prints "1"
+ *  std::cout << "b valid: " << b != nullptr << std::endl;  // prints "0"
+ * \endcode
+ *
+ * \return A valid pointer, when the containing type is of type \p T; otherwise a `nullptr`.
+ */
+template<class T>
+const T* variant_cast(const variant* operand) RTTR_NOEXCEPT;
+
+/*!
+ * \brief Returns a pointer to the containing value with type \p T.
+ *        When the containing value is of type \p T, a valid pointer to the type will be returned.
+ *        Otherwise a `nullptr` is returned.
+ *
+ * \code{.cpp}
+ *
+ *  variant var = std::string("hello world");
+ *  std:string* a = variant_cast<std::string>(&var);  // performs an internal type check and returns the value by reference
+ *  int* b        = variant_cast<int>(&var);
+ *  std::cout << "a valid: " << a != nullptr << std::endl;  // prints "1"
+ *  std::cout << "b valid: " << b != nullptr << std::endl;  // prints "0"
+ * \endcode
+ *
+ * \return A valid pointer, when the containing type is of type \p T; otherwise a `nullptr`.
+ */
+template<class T>
+T* variant_cast(variant* operand) RTTR_NOEXCEPT;
+
 } // end namespace rttr
 
 #include "rttr/detail/variant/variant_impl.h"

diff --git a/src/unit_tests/unit_tests.cmake b/src/unit_tests/unit_tests.cmake
@@ -78,6 +78,7 @@ set(SOURCE_FILES main.cpp
                  variant/variant_cmp_less_or_equal.cpp
                  variant/variant_cmp_greater_or_equal.cpp
                  variant/variant_misc_test.cpp
+                 variant/variant_cast_test.cpp
                  variant/variant_conv_to_bool.cpp
                  variant/variant_conv_to_int8.cpp
                  variant/variant_conv_to_int16.cpp