The behaviour of floating point division by zero

Question

Consider

#include <iostream>
int main()
{
    double a = 1.0 / 0;
    double b = -1.0 / 0;
    double c = 0.0 / 0;
    std::cout << a << b << c; // to stop compilers from optimising out the code.    
}

I have always thought that a will be +Inf, b will be -Inf, and c will be NaN. But I also hear rumours that strictly speaking the behaviour of floating point division by zero is undefined and therefore the above code cannot considered to be portable C++. (That theoretically obliterates the integrity of my million line plus code stack. Oops.)

Who's correct?

Note I'm happy with implementation defined, but I'm talking about cat-eating, demon-sneezing undefined behaviour here.

Answer 1

C++ standard does not force the IEEE 754 standard, because that depends mostly on hardware architecture.

If the hardware/compiler implement correctly the IEEE 754 standard, the division will provide the expected INF, -INF and NaN, otherwise... it depends.

Undefined means, the compiler implementation decides, and there are many variables to that like the hardware architecture, code generation efficiency, compiler developer laziness, etc..

Source:

The C++ standard state that a division by 0.0 is undefined

C++ Standard 5.6.4

... If the second operand of / or % is zero the behavior is undefined

C++ Standard 18.3.2.4

...static constexpr bool is_iec559;

...56. True if and only if the type adheres to IEC 559 standard.217

...57. Meaningful for all floating point types.

C++ detection of IEEE754:

The standard library includes a template to detect if IEEE754 is supported or not:

static constexpr bool is_iec559;

#include <numeric>
bool isFloatIeee754 = std::numeric_limits<float>::is_iec559();

What if IEEE754 is not supported?

It depends, usually a division by 0 trigger a hardware exception and make the application terminate.