c++googletest

ASSERT_DOUBLE_EQ fails on 2 ULPs

According to Google Test's documentation about floating point comparison, EXPECT_DOUBLE_EQ and ASSERT_DOUBLE_EQ verify that the two double values val1 and val2 are approximately equal, to within 4 ULPs (Units in the Last Place) from each other.

The following code example, however, fails the test even though the difference in the two doubles occur in 2 ULPs.

TEST(Foo, doubles_match)
{
   ASSERT_DOUBLE_EQ(0.62469504755442462, 0.62469504755442407);
}

Results:

error: Expected equality of these values:
0.62469504755442462
0.62469504755442407

What am I missing?

Solution

  • The numbers mentioned in the question are 5 ULPs apart (considering binary 64-bit IEEE-754 numbers). You can verify this with this program:

    #include <bit>
#include <cstdint>
#include <print>

int main() {
    const std::uint64_t a = std::bit_cast<std::uint64_t>(0.62469504755442462);
    const std::uint64_t b = std::bit_cast<std::uint64_t>(0.62469504755442407);
    std::print("{:016x}\n", a);
    std::print("{:016x}\n", b);
    std::print("diff: {}\n", a - b);
}

    It outputs:

    3fe3fd8077e7057a
3fe3fd8077e70575
diff: 5