Refactor json::get() to use priority_tag.

include/nlohmann/json.hpp

@@ -2838,50 +2838,53 @@ class basic_json
     /// @{
 
     /*!
-    @brief get special-case overload
+    @brief get a pointer value (implicit)
 
-    This overloads avoids a lot of template boilerplate, it can be seen as the
+    Implicit pointer access to the internally stored JSON value. No copies are
-    identity method
+    made.
 
-    @tparam BasicJsonType == @ref basic_json
+    @warning Writing data to the pointee of the result yields an undefined
+    state.
 
-    @return a copy of *this
+    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
+    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
+    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
+    assertion.
+
+    @return pointer to the internally stored JSON value if the requested
+    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
 
     @complexity Constant.
 
-    @since version 2.1.0
+    @liveexample{The example below shows how pointers to internal values of a
+    JSON value can be requested. Note that no type conversions are made and a
+    `nullptr` is returned if the value and the requested pointer type does not
+    match.,get_ptr}
+
+    @since version 1.0.0
     */
-    template<typename BasicJsonType, detail::enable_if_t<
+    template<typename PointerType, typename std::enable_if<
-                 std::is_same<typename std::remove_const<BasicJsonType>::type, basic_json_t>::value,
+                 std::is_pointer<PointerType>::value, int>::type = 0>
-                 int> = 0>
+    auto get_ptr() noexcept -> decltype(std::declval<basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
-    basic_json get() const
     {
-        return *this;
+        // delegate the call to get_impl_ptr<>()
+        return get_impl_ptr(static_cast<PointerType>(nullptr));
     }
 
     /*!
-    @brief get special-case overload
+    @brief get a pointer value (implicit)
-
+    @copydoc get_ptr()
-    This overloads converts the current @ref basic_json in a different
-    @ref basic_json type
-
-    @tparam BasicJsonType == @ref basic_json
-
-    @return a copy of *this, converted into @tparam BasicJsonType
-
-    @complexity Depending on the implementation of the called `from_json()`
-                method.
-
-    @since version 3.2.0
     */
-    template < typename BasicJsonType, detail::enable_if_t <
+    template < typename PointerType, typename std::enable_if <
-                   !std::is_same<BasicJsonType, basic_json>::value&&
+                   std::is_pointer<PointerType>::value&&
-                   detail::is_basic_json<BasicJsonType>::value, int > = 0 >
+                   std::is_const<typename std::remove_pointer<PointerType>::type>::value, int >::type = 0 >
-    BasicJsonType get() const
+    constexpr auto get_ptr() const noexcept -> decltype(std::declval<const basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
     {
-        return *this;
+        // delegate the call to get_impl_ptr<>() const
+        return get_impl_ptr(static_cast<PointerType>(nullptr));
     }
 
+  private:
     /*!
     @brief get a value (explicit)
 
@@ -2923,21 +2926,12 @@ class basic_json
     */
     template < typename ValueTypeCV, typename ValueType = detail::uncvref_t<ValueTypeCV>,
                detail::enable_if_t <
-                   !detail::is_basic_json<ValueType>::value &&
+                   detail::is_default_constructible<ValueType>::value &&
-                   detail::has_from_json<basic_json_t, ValueType>::value &&
+                   detail::has_from_json<basic_json_t, ValueType>::value,
-                   !detail::has_non_default_from_json<basic_json_t, ValueType>::value,
                    int > = 0 >
-    ValueType get() const noexcept(noexcept(
+    ValueType get_impl(detail::priority_tag<0> /*unused*/) const noexcept(noexcept(
-                                       JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>(), std::declval<ValueType&>())))
     {
-        // we cannot static_assert on ValueTypeCV being non-const, because
-        // there is support for get<const basic_json_t>(), which is why we
-        // still need the uncvref
-        static_assert(!std::is_reference<ValueTypeCV>::value,
-                      "get() cannot be used with reference types, you might want to use get_ref()");
-        static_assert(std::is_default_constructible<ValueType>::value,
-                      "types must be DefaultConstructible when used with get()");
-
         ValueType ret;
         JSONSerializer<ValueType>::from_json(*this, ret);
         return ret;
@@ -2975,15 +2969,142 @@ class basic_json
     @since version 2.1.0
     */
     template < typename ValueTypeCV, typename ValueType = detail::uncvref_t<ValueTypeCV>,
-               detail::enable_if_t < !std::is_same<basic_json_t, ValueType>::value &&
+               detail::enable_if_t <
-                                     detail::has_non_default_from_json<basic_json_t, ValueType>::value,
+                   detail::has_non_default_from_json<basic_json_t, ValueType>::value,
-                                     int > = 0 >
+                   int > = 0 >
-    ValueType get() const noexcept(noexcept(
+    ValueType get_impl(detail::priority_tag<1> /*unused*/) const noexcept(noexcept(
-                                       JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>())))
+                JSONSerializer<ValueType>::from_json(std::declval<const basic_json_t&>())))
     {
+        return JSONSerializer<ValueType>::from_json(*this);
+    }
+
+    /*!
+    @brief get special-case overload
+
+    This overloads converts the current @ref basic_json in a different
+    @ref basic_json type
+
+    @tparam BasicJsonType == @ref basic_json
+
+    @return a copy of *this, converted into @tparam BasicJsonType
+
+    @complexity Depending on the implementation of the called `from_json()`
+                method.
+
+    @since version 3.2.0
+    */
+    template < typename BasicJsonType, detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value,
+                   int > = 0 >
+    BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
+    {
+        return *this;
+    }
+
+    /*!
+    @brief get special-case overload
+
+    This overloads avoids a lot of template boilerplate, it can be seen as the
+    identity method
+
+    @tparam BasicJsonType == @ref basic_json
+
+    @return a copy of *this
+
+    @complexity Constant.
+
+    @since version 2.1.0
+    */
+    template<typename BasicJsonType, detail::enable_if_t<
+                 std::is_same<BasicJsonType, basic_json_t>::value,
+                 int> = 0>
+    basic_json get_impl(detail::priority_tag<3> /*unused*/) const
+    {
+        return *this;
+    }
+
+    /*!
+    @brief get a pointer value (explicit)
+    @copydoc get()
+    */
+    template<typename PointerType, detail::enable_if_t<
+                 std::is_pointer<PointerType>::value, int> = 0>
+    constexpr auto get_impl(detail::priority_tag<4> /*unused*/) const noexcept
+    -> decltype(std::declval<const basic_json_t&>().template get_ptr<PointerType>())
+    {
+        // delegate the call to get_ptr
+        return get_ptr<PointerType>();
+    }
+
+  public:
+    /*!
+    @brief get a (pointer) value (explicit)
+
+    Performs explicit type conversion between the JSON value and a compatible value if required.
+
+    - If the requested type is a pointer to the internally stored JSON value that pointer is returned.
+    No copies are made.
+
+    - If the requested type is the current @ref basic_json, or a different @ref basic_json convertible
+    from the current @ref basic_json.
+
+    - Otherwise the value is converted by calling the @ref json_serializer<ValueType> `from_json()`
+    method.
+
+    @tparam ValueTypeCV the provided value type
+    @tparam ValueType the returned value type
+
+    @return copy of the JSON value, converted to @tparam ValueType if necessary
+
+    @throw what @ref json_serializer<ValueType> `from_json()` method throws if conversion is required
+
+    @since version 2.1.0
+    */
+    template < typename ValueTypeCV, typename ValueType = detail::uncvref_t<ValueTypeCV>>
+    constexpr auto get() const noexcept(noexcept(get_impl<ValueType>(detail::priority_tag<4> {})))
+    -> decltype(get_impl<ValueType>(detail::priority_tag<4> {}))
+    {
+        // we cannot static_assert on ValueTypeCV being non-const, because
+        // there is support for get<const basic_json_t>(), which is why we
+        // still need the uncvref
         static_assert(!std::is_reference<ValueTypeCV>::value,
                       "get() cannot be used with reference types, you might want to use get_ref()");
-        return JSONSerializer<ValueType>::from_json(*this);
+        return get_impl<ValueType>(detail::priority_tag<4> {});
+    }
+
+    /*!
+    @brief get a pointer value (explicit)
+
+    Explicit pointer access to the internally stored JSON value. No copies are
+    made.
+
+    @warning The pointer becomes invalid if the underlying JSON object
+    changes.
+
+    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
+    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
+    @ref number_unsigned_t, or @ref number_float_t.
+
+    @return pointer to the internally stored JSON value if the requested
+    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
+
+    @complexity Constant.
+
+    @liveexample{The example below shows how pointers to internal values of a
+    JSON value can be requested. Note that no type conversions are made and a
+    `nullptr` is returned if the value and the requested pointer type does not
+    match.,get__PointerType}
+
+    @sa @ref get_ptr() for explicit pointer-member access
+
+    @since version 1.0.0
+    */
+    template<typename PointerType, typename std::enable_if<
+                 std::is_pointer<PointerType>::value, int>::type = 0>
+    auto get() noexcept -> decltype(std::declval<basic_json_t&>().template get_ptr<PointerType>())
+    {
+        // delegate the call to get_ptr
+        return get_ptr<PointerType>();
     }
 
     /*!
@@ -3056,101 +3177,6 @@ class basic_json
         return v;
     }
 
-
-    /*!
-    @brief get a pointer value (implicit)
-
-    Implicit pointer access to the internally stored JSON value. No copies are
-    made.
-
-    @warning Writing data to the pointee of the result yields an undefined
-    state.
-
-    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
-    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
-    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
-    assertion.
-
-    @return pointer to the internally stored JSON value if the requested
-    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
-
-    @complexity Constant.
-
-    @liveexample{The example below shows how pointers to internal values of a
-    JSON value can be requested. Note that no type conversions are made and a
-    `nullptr` is returned if the value and the requested pointer type does not
-    match.,get_ptr}
-
-    @since version 1.0.0
-    */
-    template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value, int>::type = 0>
-    auto get_ptr() noexcept -> decltype(std::declval<basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
-    {
-        // delegate the call to get_impl_ptr<>()
-        return get_impl_ptr(static_cast<PointerType>(nullptr));
-    }
-
-    /*!
-    @brief get a pointer value (implicit)
-    @copydoc get_ptr()
-    */
-    template < typename PointerType, typename std::enable_if <
-                   std::is_pointer<PointerType>::value&&
-                   std::is_const<typename std::remove_pointer<PointerType>::type>::value, int >::type = 0 >
-    constexpr auto get_ptr() const noexcept -> decltype(std::declval<const basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
-    {
-        // delegate the call to get_impl_ptr<>() const
-        return get_impl_ptr(static_cast<PointerType>(nullptr));
-    }
-
-    /*!
-    @brief get a pointer value (explicit)
-
-    Explicit pointer access to the internally stored JSON value. No copies are
-    made.
-
-    @warning The pointer becomes invalid if the underlying JSON object
-    changes.
-
-    @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
-    object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
-    @ref number_unsigned_t, or @ref number_float_t.
-
-    @return pointer to the internally stored JSON value if the requested
-    pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
-
-    @complexity Constant.
-
-    @liveexample{The example below shows how pointers to internal values of a
-    JSON value can be requested. Note that no type conversions are made and a
-    `nullptr` is returned if the value and the requested pointer type does not
-    match.,get__PointerType}
-
-    @sa @ref get_ptr() for explicit pointer-member access
-
-    @since version 1.0.0
-    */
-    template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value, int>::type = 0>
-    auto get() noexcept -> decltype(std::declval<basic_json_t&>().template get_ptr<PointerType>())
-    {
-        // delegate the call to get_ptr
-        return get_ptr<PointerType>();
-    }
-
-    /*!
-    @brief get a pointer value (explicit)
-    @copydoc get()
-    */
-    template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value, int>::type = 0>
-    constexpr auto get() const noexcept -> decltype(std::declval<const basic_json_t&>().template get_ptr<PointerType>())
-    {
-        // delegate the call to get_ptr
-        return get_ptr<PointerType>();
-    }
-
     /*!
     @brief get a reference value (implicit)