Clang-tidy incorrect rounding

Question

The documentation for the clang-tidy [bugprone-incorrect-roundings] check says:

The number 0.499999975 (smallest representable float number below 0.5) rounds to 1.0

As far as I can detect the smallest float number below 0.5 is 0.4999999702, not a 0.499999975. But despite of this, both numbers give me 0 values in naive rounding calculation:

#include <iostream>

int main() {
   
    const float v1 = 0.499999975;
    const float v2 = 0.4999999702;

    std::cout << (int)(v1+0.5) << "\n" 
              << (int)(v2+0.5) << "\n";
}

Am I missing something?

Answer 1

Regarding the arithmetic conversion in the standard:

6.3.1.8 Usual arithmetic conversions

...

• Otherwise, if the corresponding real type of either operand is double, the other operand is converted, without change of type domain, to a type whose corresponding real type is double.

• The values of floating operands and of the results of floating expressions may be represented in greater precision and range than that required by the type;

So in this line:

(int)(v1+0.5) 

Your v1 variable is promoted to a double precision floating point operation, that's why you get zero.

This should fix your problem:

#include <iostream>

int main() {

    const float v1 = 0.499999975f;
    const float v2 = 0.4999999702f;

    std::cout << (int)(v1 + 0.5f) << "\n" 
              << (int)(v2 + 0.5f) << "\n";
}