Fix compatibility with GCC 4.8

Closes #2045

.devcontainer/gcc48/devcontainer.json

@@ -0,0 +1,20 @@
+{
+	"name": "GCC 4.8",
+	"image": "conanio/gcc48",
+	"runArgs": [
+		"--name=ArduinoJson-gcc48"
+	],
+	"customizations": {
+		"vscode": {
+			"extensions": [
+				"ms-vscode.cmake-tools",
+				"josetr.cmake-language-support-vscode",
+				"ms-vscode.cpptools"
+			],
+			"settings": {
+				"cmake.generator": "Unix Makefiles",
+				"cmake.buildDirectory": "/tmp/build"
+			}
+		}
+	}
+}

.github/workflows/ci.yml

@@ -33,6 +33,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          - gcc: "4.8"
           - gcc: "5"
           - gcc: "6"
           - gcc: "7"
@@ -49,6 +50,7 @@ jobs:
       - name: Install
         run: |
           sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5 3B4FE6ACC0B21F32
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial main universe'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic main universe'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ focal main universe'
           sudo apt-get update

CHANGELOG.md

@@ -6,6 +6,7 @@ HEAD
 
 * Improve error messages when using `char` or `char*` (issue #2043)
 * Reduce `serializeJson()`'s size and stack usage (issue #2046)
+* Fix compatibility with GCC 4.8 (issue #2045)
 
 v7.0.2 (2024-01-19)
 ------

README.md

@@ -79,7 +79,7 @@ ArduinoJson is a C++ JSON library for Arduino and IoT (Internet Of Things).
     * [Unit test coverage close to 100%](https://coveralls.io/github/bblanchon/ArduinoJson?branch=7.x)
     * Continuously tested on
         * [Visual Studio 2017, 2019, 2022](https://ci.appveyor.com/project/bblanchon/arduinojson/branch/7.x)
-        * [GCC 5, 6, 7, 8, 9, 10, 11, 12](https://github.com/bblanchon/ArduinoJson/actions?query=workflow%3A%22Continuous+Integration%22)
+        * [GCC 4.8, 5, 6, 7, 8, 9, 10, 11, 12](https://github.com/bblanchon/ArduinoJson/actions?query=workflow%3A%22Continuous+Integration%22)
         * [Clang 3.9, 4.0, 5.0, 6.0, 7, 8, 9, 10, 11, 12, 13, 14, 15](https://github.com/bblanchon/ArduinoJson/actions?query=workflow%3A%22Continuous+Integration%22)
     * [Continuously fuzzed with Google OSS Fuzz](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:arduinojson)
     * Passes all default checks of [clang-tidy](https://releases.llvm.org/10.0.0/tools/clang/tools/extra/docs/clang-tidy/)

extras/CompileOptions.cmake

@@ -32,8 +32,18 @@ endif()
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
 	if((CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.8) AND(NOT ${COVERAGE}))
 		add_compile_options(-g -Og)
-	else()
+	else() # GCC 4.8
-		add_compile_options(-g -O0)
+		add_compile_options(
+			-g
+			-O0 # GCC 4.8 doesn't support -Og
+			-Wno-shadow  # allow the same name for a function parameter and a member functions
+			-Wp,-w  # Disable preprocessing warnings (see below)
+		)
+		# GCC 4.8 doesn't support __has_include, so we need to help him
+		add_definitions(
+			-DARDUINOJSON_ENABLE_STD_STRING=1
+			-DARDUINOJSON_ENABLE_STD_STREAM=1
+		)
 	endif()
 
 	add_compile_options(

extras/tests/Helpers/Allocators.hpp

@@ -90,6 +90,10 @@ class AllocatorLog {
       append(entry);
   }
 
+  void clear() {
+    log_.str("");
+  }
+
   void append(const AllocatorLogEntry& entry) {
     for (size_t i = 0; i < entry.count(); i++)
       log_ << entry.str() << "\n";
@@ -165,7 +169,7 @@ class SpyingAllocator : public ArduinoJson::Allocator {
   }
 
   void clearLog() {
-    log_ = AllocatorLog();
+    log_.clear();
   }
 
   const AllocatorLog& log() const {

extras/tests/Misc/TypeTraits.cpp

@@ -212,3 +212,8 @@ TEST_CASE("Polyfills/type_traits") {
     CHECK(is_enum<double>::value == false);
   }
 }
+
+TEST_CASE("is_std_string") {
+  REQUIRE(is_std_string<std::string>::value == true);
+  REQUIRE(is_std_string<EmptyClass>::value == false);
+}

src/ArduinoJson/Document/JsonDocument.hpp

@@ -30,7 +30,8 @@ class JsonDocument : public detail::VariantOperators<const JsonDocument&> {
   }
 
   // Move-constructor
-  JsonDocument(JsonDocument&& src) : JsonDocument() {
+  JsonDocument(JsonDocument&& src)
+      : JsonDocument(detail::DefaultAllocator::instance()) {
     swap(*this, src);
   }
 

src/ArduinoJson/Serialization/Writers/StdStringWriter.hpp

@@ -9,15 +9,13 @@
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-template <class...>
-using void_t = void;
-
 template <class T, typename = void>
 struct is_std_string : false_type {};
 
 template <class T>
 struct is_std_string<
-    T, void_t<decltype(T().push_back('a')), decltype(T().append(""))>>
+    T, typename enable_if<is_same<void, decltype(T().push_back('a'))>::value &&
+                          is_same<T&, decltype(T().append(""))>::value>::type>
     : true_type {};
 
 template <typename TDestination>