Fix stack overflow converting deep values between specializations (#5723)

* Fix stack overflow converting deep values between specializations Constructing a basic_json from another specialization (json to ordered_json or back, also via get<ordered_json>()) converted every container with its range constructor, which calls the converting constructor for each element. The call stack therefore grew with every nesting level, and a value nested some 30,000 levels deep overflowed it. The conversion now bounds its descent the way the copy constructor does since #5387: the first 128 levels are converted exactly as before, and below that convert_iteratively() finishes the value with an explicit stack. It builds each container bottom-up from its converted elements with the container's range constructor, so member order and keys that become equal are handled as before, and it gives a value its type only once its container exists, so an exception leaves nothing behind that cannot be destroyed. Parents (JSON_DIAGNOSTICS) and positions (JSON_DIAGNOSTIC_POSITIONS) are set for every value. Converting a null value no longer resets its positions: the constructor assigned null to a value that already was null, which swapped in the positions of the temporary. Fixes #5650. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Explain why converting null keeps positions and why next is a reference Review feedback on #5723 (gregmarr): clarify in comments that the converting constructor has already copied the positions of val, which the null case keeps like every other case, and that next must be a reference into pending so that ++next advances the stored iterator. Comments only; no code change. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Refer to recursion_depth_limit() in the convert_structured() docs The comment still named nesting_depth_limit, which #5637 removed on develop in favor of detail::recursion_depth_limit(). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Advance the pending iterator through pending.back() and shorten the null comment Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-10-03 13:10:33 +00:00 · 2026-09-30 21:36:04 +02:00
parent 3b6ae43c53
commit 7d7055ec50
6 changed files with 735 additions and 94 deletions
@@ -141,6 +141,58 @@ TEST_CASE("Better diagnostics with positions")
        check_objects(300);
    }

+    SECTION("converting keeps the positions of nested values (#5650)")
+    {
+        // Values nested deeper than the converting constructor's descent bound
+        // are converted without the call stack, on a path that has to carry the
+        // positions of every value over itself. Objects and arrays take turns,
+        // and the innermost value is null, which used to lose its positions.
+        const auto check_conversion = [](std::size_t depth)
+        {
+            CAPTURE(depth)
+
+            std::string text;
+            std::string closing;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                text += (i % 2 == 0) ? "[12, " : R"({"b":1, "a":)";
+                closing += (i % 2 == 0) ? ']' : '}';
+            }
+            text += "null";
+            text.append(closing.rbegin(), closing.rend());
+
+            const json original = json::parse(text);
+            const nlohmann::ordered_json converted = original;
+
+            const json* o = &original;
+            const nlohmann::ordered_json* c = &converted;
+            for (std::size_t level = 0; level <= depth; ++level)
+            {
+                CAPTURE(level)
+                REQUIRE(c->start_pos() == o->start_pos());
+                REQUIRE(c->end_pos() == o->end_pos());
+
+                if (level < depth)
+                {
+                    // the number beside the value nested next
+                    const json& o_number = o->is_object() ? o->at("b") : o->at(0);
+                    const nlohmann::ordered_json& c_number = c->is_object() ? c->at("b") : c->at(0);
+                    REQUIRE(c_number.start_pos() == o_number.start_pos());
+                    REQUIRE(c_number.end_pos() == o_number.end_pos());
+
+                    o = o->is_object() ? &o->at("a") : &o->at(1);
+                    c = c->is_object() ? &c->at("a") : &c->at(1);
+                }
+            }
+        };
+
+        check_conversion(1);
+        check_conversion(127);
+        check_conversion(128);
+        check_conversion(129);
+        check_conversion(300);
+    }
+
    SECTION("JSON patch add to primitive parent (#4292)")
    {
        // the JSON Patch "add" target /foo/bar/baz has a string parent