deploy: 4155e2938f

examples/basic_json_view__operator[]_json_pointer.cpp

@@ -0,0 +1,48 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+using json_pointer = nlohmann::json::json_pointer;
+
+int main()
+{
+    // a larger document; operator[] with a JSON pointer reaches straight to
+    // one deeply nested field, without ever building a tree for the rest
+    json_document doc = json_document::parse(R"(
+      {
+        "region": {
+          "servers": [
+            {"name": "eu-1", "metrics": {"cpu": 0.42}},
+            {"name": "eu-2", "metrics": {"cpu": 0.71}}
+          ]
+        }
+      }
+    )");
+
+    const auto root = doc.root();
+    std::cout << root[json_pointer("/region/servers/1/metrics/cpu")].materialize().dump() << '\n';
+
+    // a missing key or an out-of-range index along the path gives a
+    // discarded view, exactly where const json::operator[] would be
+    // undefined behavior for the same pointer
+    const auto missing = root[json_pointer("/region/servers/5/metrics/cpu")];
+    if (!missing.is_discarded())
+    {
+        std::cout << missing.materialize().dump() << '\n';
+    }
+    else
+    {
+        std::cout << "no such server\n";
+    }
+
+    // indexing into a primitive still throws, as basic_json::operator[]
+    // does for the same pointer
+    try
+    {
+        static_cast<void>(root[json_pointer("/region/servers/0/name/x")]);
+    }
+    catch (const nlohmann::json::out_of_range& e)
+    {
+        std::cout << e.what() << '\n';
+    }
+}