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Make the default of value() with a C string only if the member is missing

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basic_json_view::value(key, "text") and value(pointer, "text") built a
std::string from the default first and passed it on, so a call allocated
for the default even when the member exists, and copied it again when it
does not. They now look the member up and make the result directly: from the
member if there is one, else from the C string. Behavior, including the
exceptions, is the same.

Two calls (member found, member missing) with a 40 character default, Apple M1:
  1.6 K -> 0.7 K instructions
  Linux x86-64 Ir (500 rounds), GCC 13 / Clang 18: 2.46 M -> 2.22 M / 2.40 M -> 2.15 M

Belongs to the PR "json_view: read-side speedups".

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-22 09:30:19 +02:00
1 parent c09fa3047d
commit 69e3a7ef50
3 files changed
+36 -4
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tests/src/unit-json_view.cpp
+10
View File
@@ -1473,7 +1473,17 @@ TEST_CASE("json_view values")
CHECK(exception_of([&] { static_cast<void>(v.value("s", 0)); }) == without_path(exception_of([&] { static_cast<void>(j.value("s", 0)); })));
CHECK(exception_of([&] { static_cast<void>(v["n"].value("x", 0)); }) == without_path(exception_of([&] { static_cast<void>(j["n"].value("x", 0)); })));
CHECK(exception_of([&] { static_cast<void>(v["n"].value(json::json_pointer("/x"), 0)); }) == without_path(exception_of([&] { static_cast<void>(j["n"].value(json::json_pointer("/x"), 0)); })));
// string defaults: the member is not a string, or the value is neither an object nor an array
CHECK(exception_of([&] { static_cast<void>(v.value("n", "default")); }) == without_path(exception_of([&] { static_cast<void>(j.value("n", "default")); })));
CHECK(exception_of([&] { static_cast<void>(v["n"].value("x", "default")); }) == without_path(exception_of([&] { static_cast<void>(j["n"].value("x", "default")); })));
CHECK(exception_of([&] { static_cast<void>(v.value(json::json_pointer("/n"), "default")); }) == without_path(exception_of([&] { static_cast<void>(j.value(json::json_pointer("/n"), "default")); })));
CHECK(exception_of([&] { static_cast<void>(v["s"].value(json::json_pointer("/x"), "default")); }) == without_path(exception_of([&] { static_cast<void>(j["s"].value(json::json_pointer("/x"), "default")); })));
#endif
// (defaults and members longer than the short string optimization)
CHECK(v.value("s", "a default longer than fifteen characters") == "text");
CHECK(v.value("missing", "a default longer than fifteen characters") == "a default longer than fifteen characters");
CHECK(v.value(json::json_pointer("/s"), "a default longer than fifteen characters") == "text");
CHECK(v.value(json::json_pointer("/zz"), "a default longer than fifteen characters") == "a default longer than fifteen characters");
}
}