Change link() to shallowCopy() (issue #1343)

Instead of storing a pointer, the function copies the `VariantData`.

Benefits:
* smaller code
* no impact on programs that don't use this feature

Drawbacks:
* changes to the original variant are not always reflected on the copy
* modifying the original from the shallow copy leads to UB

extras/tests/JsonVariant/shallowCopy.cpp

@@ -0,0 +1,87 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2022, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("JsonVariant::shallowCopy()") {
+  StaticJsonDocument<1024> doc1, doc2;
+  JsonVariant variant = doc1.to<JsonVariant>();
+
+  SECTION("JsonVariant::shallowCopy(JsonDocument&)") {
+    doc2["hello"] = "world";
+
+    variant.shallowCopy(doc2);
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"world\"}");
+
+    // altering the linked document should change the result
+    doc2["hello"] = "WORLD!";
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"WORLD!\"}");
+  }
+
+  SECTION("JsonVariant::shallowCopy(MemberProxy)") {
+    doc2["obj"]["hello"] = "world";
+
+    variant.shallowCopy(doc2["obj"]);
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"world\"}");
+
+    // altering the linked document should change the result
+    doc2["obj"]["hello"] = "WORLD!";
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"WORLD!\"}");
+  }
+
+  SECTION("JsonVariant::shallowCopy(ElementProxy)") {
+    doc2[0]["hello"] = "world";
+
+    variant.shallowCopy(doc2[0]);
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"world\"}");
+
+    // altering the linked document should change the result
+    doc2[0]["hello"] = "WORLD!";
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"WORLD!\"}");
+  }
+
+  SECTION("target is unbound") {
+    JsonVariant unbound;
+    variant["hello"] = "world";
+
+    variant.shallowCopy(unbound);
+
+    CHECK(variant.isUnbound() == false);
+    CHECK(variant.isNull() == true);
+    CHECK(variant.memoryUsage() == 0);
+    CHECK(variant.size() == 0);
+  }
+
+  SECTION("variant is unbound") {
+    JsonVariant unbound;
+    doc2["hello"] = "world";
+
+    unbound.shallowCopy(doc2);
+
+    CHECK(unbound.isUnbound() == true);
+    CHECK(unbound.isNull() == true);
+    CHECK(unbound.memoryUsage() == 0);
+    CHECK(unbound.size() == 0);
+  }
+
+  SECTION("preserves owned key bit") {
+    doc2.set(42);
+
+    doc1["a"].shallowCopy(doc2);
+    doc1[std::string("b")].shallowCopy(doc2);
+
+    JsonObject::iterator it = doc1.as<JsonObject>().begin();
+
+    CHECK(it->key().isLinked() == true);
+    ++it;
+    CHECK(it->key().isLinked() == false);
+  }
+}