Make string support generic (issue #1807)

Author: bblanchon

CHANGELOG.md

@@ -6,6 +6,7 @@ HEAD
 
 * Drop support for C++98/C++03. Minimum required is C++11.
 * Remove `ARDUINOJSON_NAMESPACE`; use `ArduinoJson` instead.
+* Make string support generic (issue #1807)
 
 v6.20.1 (2023-02-08)
 -------

extras/tests/Misc/StringAdapters.cpp

@@ -3,7 +3,6 @@
 // MIT License
 
 #define ARDUINOJSON_ENABLE_PROGMEM 1
-#define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
 
 #include "custom_string.hpp"
 #include "progmem_emulation.hpp"

src/ArduinoJson/Polyfills/type_traits/declval.hpp

@@ -9,6 +9,6 @@
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
 template <typename T>
-T declval();
+T&& declval();
 
 ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Serialization/Writers/StdStringWriter.hpp

@@ -7,15 +7,17 @@
 #include <ArduinoJson/Namespace.hpp>
 #include <ArduinoJson/Polyfills/type_traits.hpp>
 
-#include <string>
-
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-template <class T>
+template <class...>
+using void_t = void;
+
+template <class T, typename = void>
 struct is_std_string : false_type {};
 
-template <class TCharTraits, class TAllocator>
-struct is_std_string<std::basic_string<char, TCharTraits, TAllocator>>
+template <class T>
+struct is_std_string<
+    T, void_t<decltype(T().push_back('a')), decltype(T().append(""))>>
     : true_type {};
 
 template <typename TDestination>
@@ -25,7 +27,7 @@ class Writer<TDestination,
   Writer(TDestination& str) : _str(&str) {}
 
   size_t write(uint8_t c) {
-    _str->operator+=(static_cast<char>(c));
+    _str->push_back(static_cast<char>(c));
     return 1;
   }
 

src/ArduinoJson/Strings/Adapters/ArduinoString.hpp

@@ -0,0 +1,51 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2023, Benoit BLANCHON
+// MIT License
+
+#pragma once
+
+#include <ArduinoJson/Strings/Adapters/RamString.hpp>
+#include <ArduinoJson/Strings/StringAdapter.hpp>
+#include <ArduinoJson/Strings/StringTraits.hpp>
+
+ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
+
+template <typename T>
+struct StringAdapter<
+    T,
+    typename enable_if<
+        (string_traits<T>::has_cstr || string_traits<T>::has_data) &&
+        (string_traits<T>::has_length || string_traits<T>::has_size)>::type> {
+  typedef SizedRamString AdaptedString;
+
+  static AdaptedString adapt(const T& s) {
+    return AdaptedString(get_data(s), get_size(s));
+  }
+
+ private:
+  template <typename U>
+  static typename enable_if<string_traits<U>::has_size, size_t>::type get_size(
+      const U& s) {
+    return s.size();
+  }
+
+  template <typename U>
+  static typename enable_if<!string_traits<U>::has_size, size_t>::type get_size(
+      const U& s) {
+    return s.length();
+  }
+
+  template <typename U>
+  static typename enable_if<string_traits<U>::has_data, const char*>::type
+  get_data(const U& s) {
+    return s.data();
+  }
+
+  template <typename U>
+  static typename enable_if<!string_traits<U>::has_data, const char*>::type
+  get_data(const U& s) {
+    return s.c_str();
+  }
+};
+
+ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Strings/Adapters/StdString.hpp

@@ -7,18 +7,7 @@
 #include <ArduinoJson/Polyfills/type_traits.hpp>
 #include <ArduinoJson/Strings/Adapters/JsonString.hpp>
 #include <ArduinoJson/Strings/Adapters/RamString.hpp>
-
-#if ARDUINOJSON_ENABLE_STD_STRING
-#  include <ArduinoJson/Strings/Adapters/StdString.hpp>
-#endif
-
-#if ARDUINOJSON_ENABLE_STRING_VIEW
-#  include <ArduinoJson/Strings/Adapters/StringView.hpp>
-#endif
-
-#if ARDUINOJSON_ENABLE_ARDUINO_STRING
-#  include <ArduinoJson/Strings/Adapters/ArduinoString.hpp>
-#endif
+#include <ArduinoJson/Strings/Adapters/StringObject.hpp>
 
 #if ARDUINOJSON_ENABLE_PROGMEM
 #  include <ArduinoJson/Strings/Adapters/FlashString.hpp>

src/ArduinoJson/Strings/Adapters/StringObject.hpp

@@ -0,0 +1,79 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2023, Benoit BLANCHON
+// MIT License
+
+#pragma once
+
+#include <ArduinoJson/Strings/Adapters/RamString.hpp>
+#include <ArduinoJson/Strings/StringAdapter.hpp>
+
+ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
+
+namespace string_traits_impl {
+
+// const char* c_str() const
+// - String
+// - std::string
+
+template <class T, class = void>
+struct has_cstr : false_type {};
+
+template <class T>
+struct has_cstr<T,
+                typename enable_if<is_same<decltype(declval<const T>().c_str()),
+                                           const char*>::value>::type>
+    : true_type {};
+
+// const char* data() const
+// - std::string
+// - std::string_view
+// - etl::string
+
+template <class T, class = void>
+struct has_data : false_type {};
+
+template <class T>
+struct has_data<T,
+                typename enable_if<is_same<decltype(declval<const T>().data()),
+                                           const char*>::value>::type>
+    : true_type {};
+
+// size_t length() const
+// - String
+
+template <class T, class = void>
+struct has_length : false_type {};
+
+template <class T>
+struct has_length<
+    T, typename enable_if<
+           is_same<decltype(declval<const T>().length()), size_t>::value>::type>
+    : true_type {};
+
+// size_t size() const
+// - std::string
+// - std::string_view
+// - etl::string
+
+template <class T, class = void>
+struct has_size : false_type {};
+
+template <class T>
+struct has_size<
+    T, typename enable_if<
+           is_same<decltype(declval<const T>().size()), size_t>::value>::type>
+    : true_type {};
+
+}  // namespace string_traits_impl
+
+template <typename T>
+struct string_traits {
+  enum {
+    has_cstr = string_traits_impl::has_cstr<T>::value,
+    has_length = string_traits_impl::has_length<T>::value,
+    has_data = string_traits_impl::has_data<T>::value,
+    has_size = string_traits_impl::has_size<T>::value
+  };
+};
+
+ARDUINOJSON_END_PRIVATE_NAMESPACE