Removed JsonVariant::as<char>() (fixes #1498)

CHANGELOG.md

@@ -1,6 +1,22 @@
 ArduinoJson: change log
 =======================
 
+HEAD
+----
+
+* Removed `JsonVariant::as<char>()` (issue #1498)
+
+> ### BREAKING CHANGE
+>
+> We cannot cast a `JsonVariant` to a `char` anymore, so the following will break:
+> ```c++
+> char age = doc["age"];  //  error: no matching function for call to 'variantAs(VariantData*&)'
+> ```
+> Instead, you must use another integral type, such as `int8_t`:
+> ```c++
+> int8_t age = doc["age"];  // OK
+> ```
+
 v6.17.3 (2021-02-15)
 -------
 

extras/tests/FailingBuilds/CMakeLists.txt

@@ -39,3 +39,7 @@ build_should_fail(write_long_long)
 
 add_executable(delete_jsondocument delete_jsondocument.cpp)
 build_should_fail(delete_jsondocument)
+
+add_executable(variant_as_char variant_as_char.cpp)
+build_should_fail(variant_as_char)
+

extras/tests/FailingBuilds/variant_as_char.cpp

@@ -0,0 +1,12 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2021
+// MIT License
+
+#include <ArduinoJson.h>
+
+// See issue #1498
+
+int main() {
+  DynamicJsonDocument doc(1024);
+  doc["dummy"].as<char>();
+}

extras/tests/JsonVariant/types.cpp

@@ -68,9 +68,6 @@ TEST_CASE("JsonVariant set()/get()") {
   SECTION("Float") {
     checkNumericType<float>();
   }
-  SECTION("Char") {
-    checkNumericType<char>();
-  }
   SECTION("SChar") {
     checkNumericType<signed char>();
   }

src/ArduinoJson/Variant/VariantAs.hpp

@@ -53,7 +53,8 @@ struct VariantConstAs<ArrayRef> {
 // ---
 
 template <typename T>
-inline typename enable_if<is_integral<T>::value && !is_same<bool, T>::value,
+inline typename enable_if<is_integral<T>::value && !is_same<bool, T>::value &&
+                              !is_same<char, T>::value,
                           T>::type
 variantAs(const VariantData* data) {
   ARDUINOJSON_ASSERT_INTEGER_TYPE_IS_SUPPORTED(T);

src/ArduinoJson/Variant/VariantRef.hpp

@@ -251,6 +251,20 @@ class VariantRef : public VariantRefBase<VariantData>,
 
   template <typename T>
   FORCE_INLINE typename VariantAs<T>::type as() const {
+    /********************************************************************
+     **                THIS IS NOT A BUG IN THE LIBRARY                **
+     **                --------------------------------                **
+     **  Get a compilation error pointing here?                        **
+     **  It doesn't mean the error *is* here.                          **
+     **  Often, it's because you try to extract the wrong value type.  **
+     **                                                                **
+     **  For example:                                                  **
+     **    char* name = doc["name"];                                   **
+     **    char age = doc["age"];                                      **
+     **  Instead, use:                                                 **
+     **    const char* name = doc["name"];                             **
+     **    int8_t age = doc["age"];                                    **
+     ********************************************************************/
     return variantAs<typename VariantAs<T>::type>(_data, _pool);
   }