Merge pull request #2872 from nlohmann/issue2865

Fix memory leak in to_json

include/nlohmann/detail/conversions/to_json.hpp

@@ -22,6 +22,13 @@ namespace detail
 // constructors //
 //////////////////
 
+/*
+ * Note all external_constructor<>::construct functions need to call
+ * j.m_value.destroy(j.m_type) to avoid a memory leak in case j contains an
+ * allocated value (e.g., a string). See bug issue
+ * https://github.com/nlohmann/json/issues/2865 for more information.
+ */
+
 template<value_t> struct external_constructor;
 
 template<>
@@ -30,6 +37,7 @@ struct external_constructor<value_t::boolean>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::boolean_t b) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::boolean;
         j.m_value = b;
         j.assert_invariant();
@@ -42,6 +50,7 @@ struct external_constructor<value_t::string>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::string_t& s)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::string;
         j.m_value = s;
         j.assert_invariant();
@@ -50,6 +59,7 @@ struct external_constructor<value_t::string>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::string_t&& s)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::string;
         j.m_value = std::move(s);
         j.assert_invariant();
@@ -60,6 +70,7 @@ struct external_constructor<value_t::string>
                              int > = 0 >
     static void construct(BasicJsonType& j, const CompatibleStringType& str)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::string;
         j.m_value.string = j.template create<typename BasicJsonType::string_t>(str);
         j.assert_invariant();
@@ -72,6 +83,7 @@ struct external_constructor<value_t::binary>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::binary_t& b)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::binary;
         j.m_value = typename BasicJsonType::binary_t(b);
         j.assert_invariant();
@@ -80,6 +92,7 @@ struct external_constructor<value_t::binary>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::binary_t&& b)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::binary;
         j.m_value = typename BasicJsonType::binary_t(std::move(b));;
         j.assert_invariant();
@@ -92,6 +105,7 @@ struct external_constructor<value_t::number_float>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::number_float_t val) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::number_float;
         j.m_value = val;
         j.assert_invariant();
@@ -104,6 +118,7 @@ struct external_constructor<value_t::number_unsigned>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::number_unsigned_t val) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::number_unsigned;
         j.m_value = val;
         j.assert_invariant();
@@ -116,6 +131,7 @@ struct external_constructor<value_t::number_integer>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::number_integer_t val) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::number_integer;
         j.m_value = val;
         j.assert_invariant();
@@ -128,6 +144,7 @@ struct external_constructor<value_t::array>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::array_t& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = arr;
         j.set_parents();
@@ -137,6 +154,7 @@ struct external_constructor<value_t::array>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = std::move(arr);
         j.set_parents();
@@ -150,6 +168,8 @@ struct external_constructor<value_t::array>
     {
         using std::begin;
         using std::end;
+
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value.array = j.template create<typename BasicJsonType::array_t>(begin(arr), end(arr));
         j.set_parents();
@@ -159,6 +179,7 @@ struct external_constructor<value_t::array>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const std::vector<bool>& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = value_t::array;
         j.m_value.array->reserve(arr.size());
@@ -174,6 +195,7 @@ struct external_constructor<value_t::array>
              enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
     static void construct(BasicJsonType& j, const std::valarray<T>& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = value_t::array;
         j.m_value.array->resize(arr.size());
@@ -192,6 +214,7 @@ struct external_constructor<value_t::object>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::object_t& obj)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::object;
         j.m_value = obj;
         j.set_parents();
@@ -201,6 +224,7 @@ struct external_constructor<value_t::object>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::object;
         j.m_value = std::move(obj);
         j.set_parents();
@@ -214,6 +238,7 @@ struct external_constructor<value_t::object>
         using std::begin;
         using std::end;
 
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::object;
         j.m_value.object = j.template create<typename BasicJsonType::object_t>(begin(obj), end(obj));
         j.set_parents();

include/nlohmann/json.hpp

@@ -1131,53 +1131,55 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             binary = create<binary_t>(std::move(value));
         }
 
-        void destroy(value_t t) noexcept
+        void destroy(value_t t)
         {
-            // flatten the current json_value to a heap-allocated stack
+            if (t == value_t::array || t == value_t::object)
-            std::vector<basic_json> stack;
+            {
+                // flatten the current json_value to a heap-allocated stack
+                std::vector<basic_json> stack;
 
-            // move the top-level items to stack
+                // move the top-level items to stack
-            if (t == value_t::array)
+                if (t == value_t::array)
-            {
-                stack.reserve(array->size());
-                std::move(array->begin(), array->end(), std::back_inserter(stack));
-            }
-            else if (t == value_t::object)
-            {
-                stack.reserve(object->size());
-                for (auto&& it : *object)
                 {
-                    stack.push_back(std::move(it.second));
+                    stack.reserve(array->size());
+                    std::move(array->begin(), array->end(), std::back_inserter(stack));
                 }
-            }
+                else
-
-            while (!stack.empty())
-            {
-                // move the last item to local variable to be processed
-                basic_json current_item(std::move(stack.back()));
-                stack.pop_back();
-
-                // if current_item is array/object, move
-                // its children to the stack to be processed later
-                if (current_item.is_array())
                 {
-                    std::move(current_item.m_value.array->begin(), current_item.m_value.array->end(),
+                    stack.reserve(object->size());
-                              std::back_inserter(stack));
+                    for (auto&& it : *object)
-
-                    current_item.m_value.array->clear();
-                }
-                else if (current_item.is_object())
-                {
-                    for (auto&& it : *current_item.m_value.object)
                     {
                         stack.push_back(std::move(it.second));
                     }
-
-                    current_item.m_value.object->clear();
                 }
 
-                // it's now safe that current_item get destructed
+                while (!stack.empty())
-                // since it doesn't have any children
+                {
+                    // move the last item to local variable to be processed
+                    basic_json current_item(std::move(stack.back()));
+                    stack.pop_back();
+
+                    // if current_item is array/object, move
+                    // its children to the stack to be processed later
+                    if (current_item.is_array())
+                    {
+                        std::move(current_item.m_value.array->begin(), current_item.m_value.array->end(), std::back_inserter(stack));
+
+                        current_item.m_value.array->clear();
+                    }
+                    else if (current_item.is_object())
+                    {
+                        for (auto&& it : *current_item.m_value.object)
+                        {
+                            stack.push_back(std::move(it.second));
+                        }
+
+                        current_item.m_value.object->clear();
+                    }
+
+                    // it's now safe that current_item get destructed
+                    // since it doesn't have any children
+                }
             }
 
             switch (t)

single_include/nlohmann/json.hpp

@@ -4480,6 +4480,13 @@ namespace detail
 // constructors //
 //////////////////
 
+/*
+ * Note all external_constructor<>::construct functions need to call
+ * j.m_value.destroy(j.m_type) to avoid a memory leak in case j contains an
+ * allocated value (e.g., a string). See bug issue
+ * https://github.com/nlohmann/json/issues/2865 for more information.
+ */
+
 template<value_t> struct external_constructor;
 
 template<>
@@ -4488,6 +4495,7 @@ struct external_constructor<value_t::boolean>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::boolean_t b) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::boolean;
         j.m_value = b;
         j.assert_invariant();
@@ -4500,6 +4508,7 @@ struct external_constructor<value_t::string>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::string_t& s)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::string;
         j.m_value = s;
         j.assert_invariant();
@@ -4508,6 +4517,7 @@ struct external_constructor<value_t::string>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::string_t&& s)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::string;
         j.m_value = std::move(s);
         j.assert_invariant();
@@ -4518,6 +4528,7 @@ struct external_constructor<value_t::string>
                              int > = 0 >
     static void construct(BasicJsonType& j, const CompatibleStringType& str)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::string;
         j.m_value.string = j.template create<typename BasicJsonType::string_t>(str);
         j.assert_invariant();
@@ -4530,6 +4541,7 @@ struct external_constructor<value_t::binary>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::binary_t& b)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::binary;
         j.m_value = typename BasicJsonType::binary_t(b);
         j.assert_invariant();
@@ -4538,6 +4550,7 @@ struct external_constructor<value_t::binary>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::binary_t&& b)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::binary;
         j.m_value = typename BasicJsonType::binary_t(std::move(b));;
         j.assert_invariant();
@@ -4550,6 +4563,7 @@ struct external_constructor<value_t::number_float>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::number_float_t val) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::number_float;
         j.m_value = val;
         j.assert_invariant();
@@ -4562,6 +4576,7 @@ struct external_constructor<value_t::number_unsigned>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::number_unsigned_t val) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::number_unsigned;
         j.m_value = val;
         j.assert_invariant();
@@ -4574,6 +4589,7 @@ struct external_constructor<value_t::number_integer>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::number_integer_t val) noexcept
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::number_integer;
         j.m_value = val;
         j.assert_invariant();
@@ -4586,6 +4602,7 @@ struct external_constructor<value_t::array>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::array_t& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = arr;
         j.set_parents();
@@ -4595,6 +4612,7 @@ struct external_constructor<value_t::array>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = std::move(arr);
         j.set_parents();
@@ -4608,6 +4626,8 @@ struct external_constructor<value_t::array>
     {
         using std::begin;
         using std::end;
+
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value.array = j.template create<typename BasicJsonType::array_t>(begin(arr), end(arr));
         j.set_parents();
@@ -4617,6 +4637,7 @@ struct external_constructor<value_t::array>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const std::vector<bool>& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = value_t::array;
         j.m_value.array->reserve(arr.size());
@@ -4632,6 +4653,7 @@ struct external_constructor<value_t::array>
              enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
     static void construct(BasicJsonType& j, const std::valarray<T>& arr)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::array;
         j.m_value = value_t::array;
         j.m_value.array->resize(arr.size());
@@ -4650,6 +4672,7 @@ struct external_constructor<value_t::object>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, const typename BasicJsonType::object_t& obj)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::object;
         j.m_value = obj;
         j.set_parents();
@@ -4659,6 +4682,7 @@ struct external_constructor<value_t::object>
     template<typename BasicJsonType>
     static void construct(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
     {
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::object;
         j.m_value = std::move(obj);
         j.set_parents();
@@ -4672,6 +4696,7 @@ struct external_constructor<value_t::object>
         using std::begin;
         using std::end;
 
+        j.m_value.destroy(j.m_type);
         j.m_type = value_t::object;
         j.m_value.object = j.template create<typename BasicJsonType::object_t>(begin(obj), end(obj));
         j.set_parents();
@@ -18166,53 +18191,55 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             binary = create<binary_t>(std::move(value));
         }
 
-        void destroy(value_t t) noexcept
+        void destroy(value_t t)
         {
-            // flatten the current json_value to a heap-allocated stack
+            if (t == value_t::array || t == value_t::object)
-            std::vector<basic_json> stack;
+            {
+                // flatten the current json_value to a heap-allocated stack
+                std::vector<basic_json> stack;
 
-            // move the top-level items to stack
+                // move the top-level items to stack
-            if (t == value_t::array)
+                if (t == value_t::array)
-            {
-                stack.reserve(array->size());
-                std::move(array->begin(), array->end(), std::back_inserter(stack));
-            }
-            else if (t == value_t::object)
-            {
-                stack.reserve(object->size());
-                for (auto&& it : *object)
                 {
-                    stack.push_back(std::move(it.second));
+                    stack.reserve(array->size());
+                    std::move(array->begin(), array->end(), std::back_inserter(stack));
                 }
-            }
+                else
-
-            while (!stack.empty())
-            {
-                // move the last item to local variable to be processed
-                basic_json current_item(std::move(stack.back()));
-                stack.pop_back();
-
-                // if current_item is array/object, move
-                // its children to the stack to be processed later
-                if (current_item.is_array())
                 {
-                    std::move(current_item.m_value.array->begin(), current_item.m_value.array->end(),
+                    stack.reserve(object->size());
-                              std::back_inserter(stack));
+                    for (auto&& it : *object)
-
-                    current_item.m_value.array->clear();
-                }
-                else if (current_item.is_object())
-                {
-                    for (auto&& it : *current_item.m_value.object)
                     {
                         stack.push_back(std::move(it.second));
                     }
-
-                    current_item.m_value.object->clear();
                 }
 
-                // it's now safe that current_item get destructed
+                while (!stack.empty())
-                // since it doesn't have any children
+                {
+                    // move the last item to local variable to be processed
+                    basic_json current_item(std::move(stack.back()));
+                    stack.pop_back();
+
+                    // if current_item is array/object, move
+                    // its children to the stack to be processed later
+                    if (current_item.is_array())
+                    {
+                        std::move(current_item.m_value.array->begin(), current_item.m_value.array->end(), std::back_inserter(stack));
+
+                        current_item.m_value.array->clear();
+                    }
+                    else if (current_item.is_object())
+                    {
+                        for (auto&& it : *current_item.m_value.object)
+                        {
+                            stack.push_back(std::move(it.second));
+                        }
+
+                        current_item.m_value.object->clear();
+                    }
+
+                    // it's now safe that current_item get destructed
+                    // since it doesn't have any children
+                }
             }
 
             switch (t)

test/src/unit-regression2.cpp

@@ -594,4 +594,30 @@ TEST_CASE("regression tests 2")
             }
         }
     }
+
+    SECTION("issue #2865 - ASAN detects memory leaks")
+    {
+        // the code below is expected to not leak memory
+        {
+            nlohmann::json o;
+            std::string s = "bar";
+
+            nlohmann::to_json(o["foo"], s);
+
+            nlohmann::json p = o;
+
+            // call to_json with a non-null JSON value
+            nlohmann::to_json(p["foo"], s);
+        }
+
+        {
+            nlohmann::json o;
+            std::string s = "bar";
+
+            nlohmann::to_json(o["foo"], s);
+
+            // call to_json with a non-null JSON value
+            nlohmann::to_json(o["foo"], s);
+        }
+    }
 }