Add JsonVariant::link() (resolves #1343)

extras/tests/JsonVariant/link.cpp

@@ -0,0 +1,77 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2022, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("JsonVariant::link()") {
+  StaticJsonDocument<1024> doc1, doc2;
+  JsonVariant variant = doc1.to<JsonVariant>();
+
+  SECTION("JsonVariant::link(JsonDocument&)") {
+    doc2["hello"] = "world";
+
+    variant.link(doc2);
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"world\"}");
+    CHECK(variant.memoryUsage() == 0);
+
+    // altering the linked document should change the result
+    doc2["hello"] = "WORLD!";
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"WORLD!\"}");
+  }
+
+  SECTION("JsonVariant::link(MemberProxy)") {
+    doc2["obj"]["hello"] = "world";
+
+    variant.link(doc2["obj"]);
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"world\"}");
+    CHECK(variant.memoryUsage() == 0);
+
+    // altering the linked document should change the result
+    doc2["obj"]["hello"] = "WORLD!";
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"WORLD!\"}");
+  }
+
+  SECTION("JsonVariant::link(ElementProxy)") {
+    doc2[0]["hello"] = "world";
+
+    variant.link(doc2[0]);
+
+    CHECK(variant.as<std::string>() == "{\"hello\":\"world\"}");
+    CHECK(variant.memoryUsage() == 0);
+
+    // 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.link(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.link(doc2);
+
+    CHECK(unbound.isUnbound() == true);
+    CHECK(unbound.isNull() == true);
+    CHECK(unbound.memoryUsage() == 0);
+    CHECK(unbound.size() == 0);
+  }
+}