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Compare integers with floats exactly instead of widening the integer (#5459)

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The mixed number arms of JSON_IMPLEMENT_OPERATOR cast the integer to
number_float_t before comparing. Past the float's mantissa that cast is lossy:
2^63-2 and 2^63-1 both round to 2^63, so each compares equal to that float
while differing from each other. Equality is therefore intransitive and the
ordering is not a strict weak ordering, which makes std::sort over such values,
or using them as keys in std::set or std::map, undefined behavior.

Compare the two exactly instead. The integer's range is a power of two the
float represents exactly, so a float outside it is ordered by magnitude alone;
inside it, truncating the float is exact, and the integer parts and then any
fractional part decide. The helper hands back a pair whose comparison with the
original operator reproduces that ordering, which keeps every operator's return
type as it was, including partial_ordering for the spaceship.

A NaN operand is returned in both members, so NaN stays false for the
relational operators and unordered for <=>. Values a float represents exactly
still compare equal, so json(1) == json(1.0) is unchanged.

Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com>
This commit is contained in:
Qatadaha Bin Matloob
2026-09-05 09:43:07 +02:00
committed by GitHub
parent 19386dd14a
commit 137a40b9aa
4 changed files with 189 additions and 8 deletions
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include/nlohmann/json.hpp
+4 -4
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@@ -3703,19 +3703,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
} \
else if (lhs_type == value_t::number_integer && rhs_type == value_t::number_float) \
{ \
return static_cast<number_float_t>(lhs.m_data.m_value.number_integer) op rhs.m_data.m_value.number_float; \
return (detail::compare_integer_with_float(lhs.m_data.m_value.number_integer, rhs.m_data.m_value.number_float)) op (static_cast<number_float_t>(0)); \
} \
else if (lhs_type == value_t::number_float && rhs_type == value_t::number_integer) \
{ \
return lhs.m_data.m_value.number_float op static_cast<number_float_t>(rhs.m_data.m_value.number_integer); \
return (static_cast<number_float_t>(0)) op (detail::compare_integer_with_float(rhs.m_data.m_value.number_integer, lhs.m_data.m_value.number_float)); \
} \
else if (lhs_type == value_t::number_unsigned && rhs_type == value_t::number_float) \
{ \
return static_cast<number_float_t>(lhs.m_data.m_value.number_unsigned) op rhs.m_data.m_value.number_float; \
return (detail::compare_integer_with_float(lhs.m_data.m_value.number_unsigned, rhs.m_data.m_value.number_float)) op (static_cast<number_float_t>(0)); \
} \
else if (lhs_type == value_t::number_float && rhs_type == value_t::number_unsigned) \
{ \
return lhs.m_data.m_value.number_float op static_cast<number_float_t>(rhs.m_data.m_value.number_unsigned); \
return (static_cast<number_float_t>(0)) op (detail::compare_integer_with_float(rhs.m_data.m_value.number_unsigned, lhs.m_data.m_value.number_float)); \
} \
else if (lhs_type == value_t::number_unsigned && rhs_type == value_t::number_integer) \
{ \