The behaviour of floating point division by zero

Question

Consider

#include 
int main()
{
    double a = 1.0 / 0;
    double b = -1.0 / 0;
    double c = 0.0 / 0;
    std::cout << a << b          


        

Answer 1

Quoting cppreference:

If the second operand is zero, the behavior is undefined, except that if floating-point division is taking place and the type supports IEEE floating-point arithmetic (see std::numeric_limits::is_iec559), then:

• if one operand is NaN, the result is NaN

• dividing a non-zero number by ±0.0 gives the correctly-signed infinity and FE_DIVBYZERO is raised

• dividing 0.0 by 0.0 gives NaN and FE_INVALID is raised

We are talking about floating-point division here, so it is actually implementation-defined whether double division by zero is undefined.

If std::numeric_limits::is_iec559 is true, and it is "usually true", then the behaviour is well-defined and produces the expected results.

A pretty safe bet would be to plop down a:

static_assert(std::numeric_limits::is_iec559, "Please use IEEE754, you weirdo");

... near your code.