Add editable json_documents: set, push_back, insert, and erase

basic_json_document gets a second template parameter, Editable
(false by default), plus the aliases json_editable_document,
json_editable_view, ordered_json_editable_document and
ordered_json_editable_view.

Editable documents can change values and structure without
rewriting the source text: set()/push_back() on values, keys,
array indices and JSON pointers; insert() before an array
element; erase() of an object key, array index or JSON pointer.

New values and element sequences go into edit storage that the
document owns and never moves, so views keep referring to their
value across edits and a parsed node never moves. Read-only
documents walk the plain node array and are unaffected.

Strings are checked for UTF-8 on entry, so dump() of an editable
document never throws type_error.316. Binary values cannot be
stored (type_error.319).

A seeded differential test applies random edits to an editable
document and to the equivalent ordered_json and compares both
after every step.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/examples/basic_json_document__erase.cpp

@@ -0,0 +1,38 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json = nlohmann::json;
+using json_editable_document = nlohmann::json_editable_document;
+using json_editable_view = nlohmann::json_editable_view;
+
+int main()
+{
+    // a deprecated field is dropped from a configuration file, and a
+    // decommissioned replica is removed from the list -- "price" keeps its
+    // trailing zero, and the fields around the removed ones keep their order
+    const std::string text = R"({
+  "name": "cache",
+  "legacy_host": "db0",
+  "host": "db1",
+  "price": 19.90,
+  "replicas": ["db2", "db3", "db4"]
+})";
+
+    json_editable_document doc = json_editable_document::parse(text);
+
+    doc.erase(doc.root(), "legacy_host");                                      // (1) an object member
+    doc.erase(doc.root()["replicas"], 1);                                      // (2) an array element ("db3")
+    const std::size_t removed = doc.erase(json::json_pointer("/replicas/0"));  // (3) via a JSON pointer
+
+    std::cout << removed << '\n';
+    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << "\n\n";
+
+    // the same edits on a plain json value: object_t is a std::map, so
+    // parsing already sorted the keys, and dump() rewrites every number to
+    // its shortest form, even "price", which was never touched
+    json plain = json::parse(text);
+    plain.erase("legacy_host");
+    plain["replicas"].erase(1);
+    plain["replicas"].erase(0);
+    std::cout << plain.dump(2) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_document__erase.output

@@ -0,0 +1,18 @@
+1
+{
+  "name": "cache",
+  "host": "db1",
+  "price": 19.90,
+  "replicas": [
+    "db4"
+  ]
+}
+
+{
+  "host": "db1",
+  "name": "cache",
+  "price": 19.9,
+  "replicas": [
+    "db4"
+  ]
+}

docs/mkdocs/docs/examples/basic_json_document__insert.cpp

@@ -0,0 +1,38 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json = nlohmann::json;
+using json_editable_document = nlohmann::json_editable_document;
+using json_editable_view = nlohmann::json_editable_view;
+
+int main()
+{
+    // a deployment plan -- "budget" is written with a trailing zero that has
+    // no effect on its value
+    const std::string text = R"({
+  "release": "2026.09",
+  "steps": ["build", "test", "deploy"],
+  "budget": 19.90
+})";
+
+    json_editable_document doc = json_editable_document::parse(text);
+
+    const std::size_t deploy_index = 2;
+    const auto deploy = doc.root()["steps"][deploy_index];  // held across the insert
+
+    doc.insert(doc.root()["steps"], deploy_index, "smoke-test"); // insert before "deploy"
+
+    // the held view still refers to "deploy", even though its index moved
+    // from 2 to 3, and nothing else in the document was touched
+    std::cout << deploy.dump() << '\n';
+    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << "\n\n";
+
+    // the same edit on a plain json value: an index held from before the
+    // insert now refers to whatever moved into that slot, and dump()
+    // rewrites "budget" to its shortest form even though it was never
+    // touched
+    json plain = json::parse(text);
+    plain["steps"].insert(plain["steps"].begin() + static_cast<std::ptrdiff_t>(deploy_index), "smoke-test");
+    std::cout << plain["steps"][deploy_index].dump() << '\n';
+    std::cout << plain.dump(2) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_document__insert.output

@@ -0,0 +1,23 @@
+"deploy"
+{
+  "release": "2026.09",
+  "steps": [
+    "build",
+    "test",
+    "smoke-test",
+    "deploy"
+  ],
+  "budget": 19.90
+}
+
+"smoke-test"
+{
+  "budget": 19.9,
+  "release": "2026.09",
+  "steps": [
+    "build",
+    "test",
+    "smoke-test",
+    "deploy"
+  ]
+}

docs/mkdocs/docs/examples/basic_json_document__push_back.cpp

@@ -0,0 +1,24 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json = nlohmann::json;
+using json_editable_document = nlohmann::json_editable_document;
+
+int main()
+{
+    // "events" starts out null -- the first push_back() turns it into an
+    // array, exactly like set() turns a null object member into an object
+    json_editable_document doc = json_editable_document::parse(R"({"source": "sensor-1", "events": null})");
+
+    const auto first = doc.push_back(doc.root()["events"], json{{"type", "start"}, {"t", 0}});
+    for (int t = 1; t <= 3; ++t)
+    {
+        doc.push_back(doc.root()["events"], json{{"type", "tick"}, {"t", t}});
+    }
+
+    // push_back() never moves an existing element: a view taken from an
+    // earlier call still refers to the same element after later ones
+    std::cout << first.dump() << '\n';
+    std::cout << doc.root()["events"].size() << '\n';
+    std::cout << doc.root().dump(2) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_document__push_back.output

@@ -0,0 +1,23 @@
+{"t":0,"type":"start"}
+4
+{
+  "source": "sensor-1",
+  "events": [
+    {
+      "t": 0,
+      "type": "start"
+    },
+    {
+      "t": 1,
+      "type": "tick"
+    },
+    {
+      "t": 2,
+      "type": "tick"
+    },
+    {
+      "t": 3,
+      "type": "tick"
+    }
+  ]
+}

docs/mkdocs/docs/examples/basic_json_document__set.cpp

@@ -0,0 +1,41 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json = nlohmann::json;
+using json_editable_document = nlohmann::json_editable_document;
+using json_editable_view = nlohmann::json_editable_view;
+
+int main()
+{
+    // a configuration file, as it might be read from disk -- "price" is
+    // written with a trailing zero that has no effect on its value
+    const std::string text = R"({
+  "name": "cache",
+  "host": "db1",
+  "port": 6379,
+  "price": 19.90,
+  "replicas": ["db2", "db3"],
+  "timeout": 30
+})";
+
+    json_editable_document doc = json_editable_document::parse(text);
+
+    doc.set(doc.root()["port"], 6380);                    // (1) replace a value
+    doc.set(doc.root(), "region", "us-east");              // (2) add a member
+    doc.set(doc.root()["replicas"], 0, "db4");             // (3) assign an element
+    doc.set(json::json_pointer("/timeout"), 45);           // (4) via a JSON pointer
+
+    // members stay in document order (the new one at the end), and a number
+    // that was not itself edited keeps its exact spelling
+    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << "\n\n";
+
+    // the same edits on a plain json value: object_t is a std::map, so
+    // parsing already sorted the keys, and dump() rewrites every number to
+    // its shortest form, even "price", which was never touched
+    json plain = json::parse(text);
+    plain["port"] = 6380;
+    plain["region"] = "us-east";
+    plain["replicas"][0] = "db4";
+    plain[json::json_pointer("/timeout")] = 45;
+    std::cout << plain.dump(2) << '\n';
+}

docs/mkdocs/docs/examples/basic_json_document__set.output

@@ -0,0 +1,25 @@
+{
+  "name": "cache",
+  "host": "db1",
+  "port": 6380,
+  "price": 19.90,
+  "replicas": [
+    "db4",
+    "db3"
+  ],
+  "timeout": 45,
+  "region": "us-east"
+}
+
+{
+  "host": "db1",
+  "name": "cache",
+  "port": 6380,
+  "price": 19.9,
+  "region": "us-east",
+  "replicas": [
+    "db4",
+    "db3"
+  ],
+  "timeout": 45
+}

docs/mkdocs/docs/examples/json_editable_document.cpp

@@ -0,0 +1,30 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json = nlohmann::json;
+using json_editable_document = nlohmann::json_editable_document;
+using json_editable_view = nlohmann::json_editable_view;
+
+int main()
+{
+    // a configuration file, as it might be read from disk
+    const std::string text = R"({"name": "cache", "host": "db1", "port": 6379, "price": 19.90})";
+    std::cout << text << "\n\n";
+
+    // patch two fields -- "price" is never touched
+    json_editable_document doc = json_editable_document::parse(text);
+    doc.set(doc.root(), "host", "db2");
+    doc.set(doc.root(), "retries", 3);
+
+    // member order (the new member at the end) and the untouched number's
+    // exact spelling survive
+    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\n';
+
+    // the same patch on a plain json value: keys are sorted (object_t is a
+    // std::map), and "price" is rewritten even though the patch never
+    // touched it
+    json plain = json::parse(text);
+    plain["host"] = "db2";
+    plain["retries"] = 3;
+    std::cout << plain.dump() << '\n';
+}

docs/mkdocs/docs/examples/json_editable_document.output

@@ -0,0 +1,4 @@
+{"name": "cache", "host": "db1", "port": 6379, "price": 19.90}
+
+{"name":"cache","host":"db2","port":6379,"price":19.90,"retries":3}
+{"host":"db2","name":"cache","port":6379,"price":19.9,"retries":3}

docs/mkdocs/docs/examples/ordered_json_editable_document.cpp

@@ -0,0 +1,15 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using ordered_json_editable_document = nlohmann::ordered_json_editable_document;
+
+int main()
+{
+    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, true>
+    ordered_json_editable_document doc = ordered_json_editable_document::parse(R"({"z": 1, "a": 2, "m": 3})");
+    doc.set(doc.root(), "b", 4); // set() always appends a new member at the end
+
+    // materialize() preserves the document order (with "b" at the end),
+    // instead of sorting the keys the way json_editable_document does
+    std::cout << doc.root().materialize().dump() << '\n';
+}

docs/mkdocs/docs/examples/ordered_json_editable_document.output

@@ -0,0 +1 @@
+{"z":1,"a":2,"m":3,"b":4}