✨ make emplace_back return a reference #1609

nlohmann · Jun 30, 2019 · 1be6343 · 1be6343
1 parent 9a775bc
commit 1be6343
Show file tree

Hide file tree

Showing 3 changed files with 26 additions and 8 deletions.
diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp
@@ -4952,6 +4952,8 @@ class basic_json
     @param[in] args arguments to forward to a constructor of @ref basic_json
     @tparam Args compatible types to create a @ref basic_json object
 
+    @return reference to the inserted element
+
     @throw type_error.311 when called on a type other than JSON array or
     null; example: `"cannot use emplace_back() with number"`
 
@@ -4961,10 +4963,10 @@ class basic_json
     elements to a JSON array. Note how the `null` value was silently converted
     to a JSON array.,emplace_back}
 
-    @since version 2.0.8
+    @since version 2.0.8, returns reference since 3.7.0
     */
     template<class... Args>
-    void emplace_back(Args&& ... args)
+    reference emplace_back(Args&& ... args)
     {
         // emplace_back only works for null objects or arrays
         if (JSON_UNLIKELY(not(is_null() or is_array())))
@@ -4981,7 +4983,12 @@ class basic_json
         }
 
         // add element to array (perfect forwarding)
+#ifdef JSON_HAS_CPP_17
+        return m_value.array->emplace_back(std::forward<Args>(args)...);
+#else
         m_value.array->emplace_back(std::forward<Args>(args)...);
+        return m_value.array->back();
+#endif
     }
 
     /*!

diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
@@ -17763,6 +17763,8 @@ class basic_json
     @param[in] args arguments to forward to a constructor of @ref basic_json
     @tparam Args compatible types to create a @ref basic_json object
 
+    @return reference to the inserted element
+
     @throw type_error.311 when called on a type other than JSON array or
     null; example: `"cannot use emplace_back() with number"`
 
@@ -17772,10 +17774,10 @@ class basic_json
     elements to a JSON array. Note how the `null` value was silently converted
     to a JSON array.,emplace_back}
 
-    @since version 2.0.8
+    @since version 2.0.8, returns reference since 3.7.0
     */
     template<class... Args>
-    void emplace_back(Args&& ... args)
+    reference emplace_back(Args&& ... args)
     {
         // emplace_back only works for null objects or arrays
         if (JSON_UNLIKELY(not(is_null() or is_array())))
@@ -17792,7 +17794,12 @@ class basic_json
         }
 
         // add element to array (perfect forwarding)
+#ifdef JSON_HAS_CPP_17
+        return m_value.array->emplace_back(std::forward<Args>(args)...);
+#else
         m_value.array->emplace_back(std::forward<Args>(args)...);
+        return m_value.array->back();
+#endif
     }
 
     /*!

diff --git a/test/src/unit-modifiers.cpp b/test/src/unit-modifiers.cpp
@@ -290,24 +290,28 @@ TEST_CASE("modifiers")
             SECTION("null")
             {
                 json j;
-                j.emplace_back(1);
-                j.emplace_back(2);
+                auto& x1 = j.emplace_back(1);
+                CHECK(x1 == 1);
+                auto& x2 = j.emplace_back(2);
+                CHECK(x2 == 2);
                 CHECK(j.type() == json::value_t::array);
                 CHECK(j == json({1, 2}));
             }
 
             SECTION("array")
             {
                 json j = {1, 2, 3};
-                j.emplace_back("Hello");
+                auto& x = j.emplace_back("Hello");
+                CHECK(x == "Hello");
                 CHECK(j.type() == json::value_t::array);
                 CHECK(j == json({1, 2, 3, "Hello"}));
             }
 
             SECTION("multiple values")
             {
                 json j;
-                j.emplace_back(3, "foo");
+                auto& x = j.emplace_back(3, "foo");
+                CHECK(x == json({"foo", "foo", "foo"}));
                 CHECK(j.type() == json::value_t::array);
                 CHECK(j == json({{"foo", "foo", "foo"}}));
             }