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Make comparisons with scalars noexcept only when the conversion is

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The comparison operators taking a scalar (==, !=, <, <=, >, >=, and
C++20's <=>) convert the scalar to a basic_json and compare, but were
unconditionally noexcept. When that conversion throws, the program
called std::terminate instead of propagating the exception, e.g. when
comparing a json with a string literal under memory pressure
(std::bad_alloc) or with an enum value not mapped by
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT (out_of_range.410). clang-tidy
22.1 reports the latter as bugprone-exception-escape.

Declare the 16 scalar overloads
noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value):
they stay noexcept for numbers, Booleans, nullptr, and plain enums,
and are noexcept(false) for strings and enums whose to_json may throw.
The comparisons of two basic_json values are unchanged.

Restore the strict-enum comparisons removed from unit-conversions.cpp
in the previous PR, check that comparing an unmapped strict enum now
throws, and pin the new exception specifications in unit-noexcept.cpp.
Document the exception safety of overload (2) on all seven operator
pages. Ran make amalgamate.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-10-02 11:23:58 +02:00
committed by GitHub
parent 4d46bce4e1
commit c0694f7966
11 changed files with 126 additions and 54 deletions
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tests/src/unit-noexcept.cpp
+9
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@@ -54,6 +54,15 @@ static_assert(noexcept(json(pod {})), "");
static_assert(noexcept(std::declval<json>().get<pod>()), "");
static_assert(!noexcept(std::declval<json>().get<pod_bis>()), "");
static_assert(noexcept(json(pod{})), "");
// comparing with a scalar is noexcept exactly when converting the scalar is
static_assert(noexcept(std::declval<const json&>() == 1), "");
static_assert(noexcept(1 != std::declval<const json&>()), "");
static_assert(noexcept(std::declval<const json&>() < 2.5), "");
static_assert(noexcept(nullptr == std::declval<const json&>()), "");
static_assert(!noexcept(std::declval<const json&>() == "foo"), "");
static_assert(!noexcept("foo" >= std::declval<const json&>()), "");
static_assert(noexcept(std::declval<const json&>() == std::declval<const json&>()), "");
} // namespace
TEST_CASE("noexcept")