C++ warning: division of double by zero

Case 1:

#include <iostream>

int main()
{
    double d = 15.50;
    std::cout<<(d/0.0)<<std::endl;
}

It compiles without any warnings and prints inf. OK, C++ can handle division by zero, (see it live).

But,

Case 2:

#include <iostream>

int main()
{
    double d = 15.50;
    std::cout<<(d/0)<<std::endl;
}

The compiler gives the following warning (see it live):

warning: division by zero [-Wdiv-by-zero]
     std::cout<<(d/0)<<std::endl;

Why does the compiler give a warning in the second case?

Is 0 != 0.0?

Edit:

#include <iostream>

int main()
{
    if(0 == 0.0)
        std::cout<<"Same"<<std::endl;
    else
        std::cout<<"Not same"<<std::endl;
}

output:

Same

Solution 1:

Floating point division by zero is well defined by IEEE and gives infinity (either positive or negative according to the value of the numerator ^{(or NaN for ±0)} ).

For integers, there is no way to represent infinity and the language defines the operation to have undefined behaviour so the compiler helpfully tries to steer you clear from that path.

However in this case, since the numerator is a double, the divisor (0) should be promoted to a double too and there's no reason to give a warning here whilst not giving a warning for 0.0 so I think this is a compiler bug.

Solution 2:

In Standard C++, both cases are undefined behaviour. Anything may happen, including formatting your hard drive. You should not expect or rely on "return inf. Ok" or any other behaviour.

The compiler apparently decides to give a warning in one case and not the other, but this doesn't mean that one code is OK and one isn't. It is just a quirk of the compiler's generation of warnings.

From the C++17 standard [expr.mul]/4:

The binary / operator yields the quotient, and the binary % operator yields the remainder from the division of the first expression by the second. If the second operand of / or % is zero the behavior is undefined.