From 40a14c4ed8e3f75a9e844c79f51f93a7d247c4ed Mon Sep 17 00:00:00 2001 From: Niels Lohmann Date: Wed, 2 Sep 2026 23:34:39 +0200 Subject: [PATCH] Move the scanned string into the value instead of copying it The SAX interface documents that the string handed to json_sax::string() may be moved from, and the DOM handlers already move the one handed to binary(). string() did not, so every string value was copy-constructed out of the lexer's token buffer, which then kept the buffer alive at its high-water mark until the next token overwrote it. Moving hands that buffer to the new value instead. The allocation count is unchanged - the value needed one either way - but the copy is gone. jeopardy 247.3 ms -> 240.9 ms (-2.6%) citm_catalog 4.61 ms -> 4.48 ms (-2.8%) 40k 30-char strings 7.80 ms -> 7.64 ms (-2.1%) Note this deliberately does not extend to the object key. Moving the key hands the lexer's buffer - sized for the largest token seen so far - to a key that is usually short, so the next value has to grow a fresh buffer. Measured, that costs 11.9% on a document of many small keys with longer values. Signed-off-by: Niels Lohmann --- include/nlohmann/detail/input/json_sax.hpp | 7 +++++-- single_include/nlohmann/json.hpp | 7 +++++-- 2 files changed, 10 insertions(+), 4 deletions(-) diff --git a/include/nlohmann/detail/input/json_sax.hpp b/include/nlohmann/detail/input/json_sax.hpp index 684fac047..daad030ab 100644 --- a/include/nlohmann/detail/input/json_sax.hpp +++ b/include/nlohmann/detail/input/json_sax.hpp @@ -222,7 +222,9 @@ class json_sax_dom_parser bool string(string_t& val) { - handle_value(val); + // the interface allows moving the value (see json_sax::string), which + // hands the lexer's buffer to the new value instead of copying it + handle_value(std::move(val)); return true; } @@ -532,7 +534,8 @@ class json_sax_dom_callback_parser bool string(string_t& val) { - handle_value(val); + // see json_sax_dom_parser::string() + handle_value(std::move(val)); return true; } diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp index b8da1e526..1a16ff705 100644 --- a/single_include/nlohmann/json.hpp +++ b/single_include/nlohmann/json.hpp @@ -10739,7 +10739,9 @@ class json_sax_dom_parser bool string(string_t& val) { - handle_value(val); + // the interface allows moving the value (see json_sax::string), which + // hands the lexer's buffer to the new value instead of copying it + handle_value(std::move(val)); return true; } @@ -11049,7 +11051,8 @@ class json_sax_dom_callback_parser bool string(string_t& val) { - handle_value(val); + // see json_sax_dom_parser::string() + handle_value(std::move(val)); return true; }