re-adding const version operator[] (#135, #159)

It was a good idea to implement a const version of operator[] it in the
first place. I was a pity that this implementation was flawed. It was a
mistake to remove the const version completely. This commit
re-introduces the const version. My apologies for all the inconvenience.

doc/examples/operatorarray__key_type_const.cpp

@@ -5,7 +5,7 @@ using namespace nlohmann;
 int main()
 {
     // create a JSON object
-    json object =
+    const json object =
     {
         {"one", 1}, {"two", 2}, {"three", 2.9}
     };

doc/examples/operatorarray__key_type_const.link

-<a target="_blank" href="http://melpon.org/wandbox/permlink/9X5Q8p7DlWA6v0Ey"><b>online</b></a>
\ No newline at end of file
+<a target="_blank" href="http://melpon.org/wandbox/permlink/4WLxv7id8P64Q1KI"><b>online</b></a>
\ No newline at end of file

src/json.hpp

@@ -2862,6 +2862,45 @@ class basic_json
         }
     }
 
+    /*!
+    @brief read-only access specified object element
+
+    Returns a const reference to the element at with specified key @a key. No
+    bounds checking is performed.
+
+    @warning If the element with key @a key does not exist, the behavior is
+    undefined.
+
+    @param[in] key  key of the element to access
+
+    @return const reference to the element at key @a key
+
+    @throw std::domain_error if JSON is not an object
+
+    @complexity Logarithmic in the size of the container.
+
+    @liveexample{The example below shows how object elements can be read using
+    the [] operator.,operatorarray__key_type_const}
+
+    @sa @ref at(const typename object_t::key_type&) for access by reference
+    with range checking
+    @sa @ref value() for access by value with a default value
+
+    @since version 1.0
+    */
+    const_reference operator[](const typename object_t::key_type& key) const
+    {
+        // [] only works for objects
+        if (is_object())
+        {
+            return m_value.object->find(key)->second;
+        }
+        else
+        {
+            throw std::domain_error("cannot use operator[] with " + type_name());
+        }
+    }
+
     /*!
     @brief access specified object element
 
@@ -2911,6 +2950,48 @@ class basic_json
         }
     }
 
+    /*!
+    @brief read-only access specified object element
+
+    Returns a const reference to the element at with specified key @a key. No
+    bounds checking is performed.
+
+    @warning If the element with key @a key does not exist, the behavior is
+    undefined.
+
+    @note This function is required for compatibility reasons with Clang.
+
+    @param[in] key  key of the element to access
+
+    @return const reference to the element at key @a key
+
+    @throw std::domain_error if JSON is not an object
+
+    @complexity Logarithmic in the size of the container.
+
+    @liveexample{The example below shows how object elements can be read using
+    the [] operator.,operatorarray__key_type_const}
+
+    @sa @ref at(const typename object_t::key_type&) for access by reference
+    with range checking
+    @sa @ref value() for access by value with a default value
+
+    @since version 1.0
+    */
+    template<typename T, std::size_t n>
+    const_reference operator[](const T (&key)[n]) const
+    {
+        // at only works for objects
+        if (is_object())
+        {
+            return m_value.object->find(key)->second;
+        }
+        else
+        {
+            throw std::domain_error("cannot use operator[] with " + type_name());
+        }
+    }
+
     /*!
     @brief access specified object element with default value
 

src/json.hpp.re2c

@@ -2862,6 +2862,45 @@ class basic_json
         }
     }
 
+    /*!
+    @brief read-only access specified object element
+
+    Returns a const reference to the element at with specified key @a key. No
+    bounds checking is performed.
+
+    @warning If the element with key @a key does not exist, the behavior is
+    undefined.
+
+    @param[in] key  key of the element to access
+
+    @return const reference to the element at key @a key
+
+    @throw std::domain_error if JSON is not an object
+
+    @complexity Logarithmic in the size of the container.
+
+    @liveexample{The example below shows how object elements can be read using
+    the [] operator.,operatorarray__key_type_const}
+
+    @sa @ref at(const typename object_t::key_type&) for access by reference
+    with range checking
+    @sa @ref value() for access by value with a default value
+
+    @since version 1.0
+    */
+    const_reference operator[](const typename object_t::key_type& key) const
+    {
+        // [] only works for objects
+        if (is_object())
+        {
+            return m_value.object->find(key)->second;
+        }
+        else
+        {
+            throw std::domain_error("cannot use operator[] with " + type_name());
+        }
+    }
+
     /*!
     @brief access specified object element
 
@@ -2911,6 +2950,48 @@ class basic_json
         }
     }
 
+    /*!
+    @brief read-only access specified object element
+
+    Returns a const reference to the element at with specified key @a key. No
+    bounds checking is performed.
+
+    @warning If the element with key @a key does not exist, the behavior is
+    undefined.
+
+    @note This function is required for compatibility reasons with Clang.
+
+    @param[in] key  key of the element to access
+
+    @return const reference to the element at key @a key
+
+    @throw std::domain_error if JSON is not an object
+
+    @complexity Logarithmic in the size of the container.
+
+    @liveexample{The example below shows how object elements can be read using
+    the [] operator.,operatorarray__key_type_const}
+
+    @sa @ref at(const typename object_t::key_type&) for access by reference
+    with range checking
+    @sa @ref value() for access by value with a default value
+
+    @since version 1.0
+    */
+    template<typename T, std::size_t n>
+    const_reference operator[](const T (&key)[n]) const
+    {
+        // at only works for objects
+        if (is_object())
+        {
+            return m_value.object->find(key)->second;
+        }
+        else
+        {
+            throw std::domain_error("cannot use operator[] with " + type_name());
+        }
+    }
+
     /*!
     @brief access specified object element with default value
 

test/unit.cpp

@@ -3211,6 +3211,27 @@ TEST_CASE("element access")
 
                 CHECK(j["array"] == json({1, 2, 3}));
                 CHECK(j[json::object_t::key_type("array")] == j["array"]);
+
+                CHECK(j_const["integer"] == json(1));
+                CHECK(j_const[json::object_t::key_type("integer")] == j["integer"]);
+
+                CHECK(j_const["boolean"] == json(true));
+                CHECK(j_const[json::object_t::key_type("boolean")] == j["boolean"]);
+
+                CHECK(j_const["null"] == json(nullptr));
+                CHECK(j_const[json::object_t::key_type("null")] == j["null"]);
+
+                CHECK(j_const["string"] == json("hello world"));
+                CHECK(j_const[json::object_t::key_type("string")] == j["string"]);
+
+                CHECK(j_const["floating"] == json(42.23));
+                CHECK(j_const[json::object_t::key_type("floating")] == j["floating"]);
+
+                CHECK(j_const["object"] == json(json::object()));
+                CHECK(j_const[json::object_t::key_type("object")] == j["object"]);
+
+                CHECK(j_const["array"] == json({1, 2, 3}));
+                CHECK(j_const[json::object_t::key_type("array")] == j["array"]);
             }
 
             SECTION("access on non-object type")
@@ -3222,6 +3243,8 @@ TEST_CASE("element access")
                     const json j_const_nonobject(j_nonobject);
                     CHECK_NOTHROW(j_nonobject["foo"]);
                     CHECK_NOTHROW(j_nonobject2[json::object_t::key_type("foo")]);
+                    CHECK_THROWS_AS(j_const_nonobject["foo"], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject[json::object_t::key_type("foo")], std::domain_error);
                 }
 
                 SECTION("boolean")
@@ -3230,6 +3253,8 @@ TEST_CASE("element access")
                     const json j_const_nonobject(j_nonobject);
                     CHECK_THROWS_AS(j_nonobject["foo"], std::domain_error);
                     CHECK_THROWS_AS(j_nonobject[json::object_t::key_type("foo")], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject["foo"], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject[json::object_t::key_type("foo")], std::domain_error);
                 }
 
                 SECTION("string")
@@ -3238,6 +3263,8 @@ TEST_CASE("element access")
                     const json j_const_nonobject(j_nonobject);
                     CHECK_THROWS_AS(j_nonobject["foo"], std::domain_error);
                     CHECK_THROWS_AS(j_nonobject[json::object_t::key_type("foo")], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject["foo"], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject[json::object_t::key_type("foo")], std::domain_error);
                 }
 
                 SECTION("array")
@@ -3246,6 +3273,8 @@ TEST_CASE("element access")
                     const json j_const_nonobject(j_nonobject);
                     CHECK_THROWS_AS(j_nonobject["foo"], std::domain_error);
                     CHECK_THROWS_AS(j_nonobject[json::object_t::key_type("foo")], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject["foo"], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject[json::object_t::key_type("foo")], std::domain_error);
                 }
 
                 SECTION("number (integer)")
@@ -3254,6 +3283,8 @@ TEST_CASE("element access")
                     const json j_const_nonobject(j_nonobject);
                     CHECK_THROWS_AS(j_nonobject["foo"], std::domain_error);
                     CHECK_THROWS_AS(j_nonobject[json::object_t::key_type("foo")], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject["foo"], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject[json::object_t::key_type("foo")], std::domain_error);
                 }
 
                 SECTION("number (floating-point)")
@@ -3262,6 +3293,8 @@ TEST_CASE("element access")
                     const json j_const_nonobject(j_nonobject);
                     CHECK_THROWS_AS(j_nonobject["foo"], std::domain_error);
                     CHECK_THROWS_AS(j_nonobject[json::object_t::key_type("foo")], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject["foo"], std::domain_error);
+                    CHECK_THROWS_AS(j_const_nonobject[json::object_t::key_type("foo")], std::domain_error);
                 }
             }
         }