why does g++ not warn about division by zero?

[Martin Ettl]

Hello Axel,

i played around with the suggested option   -Wno-div-by-zero, but g++ does not warn about the divsion by zero. I tested the following:

foo(4./0)   // float/integer
foo(4/0)    // integer/integer
foo(4/0.)   // integer/float
foo(4./0.)  // float/float

g++ does not warn about this cases. Here the extended testcase:


#include <iostream>

template <typename T1, typename T2> inline T1 foo (T1 const& a, T2 const& b)
{
	// ....
    return a/b;
}

int main()
{
	std::cout << foo(4.,0) << std::endl; 
	std::cout << foo(4,0.) << std::endl; 
	std::cout << foo(4,0) << std::endl; 
	std::cout << foo(4.,0.) << std::endl; 
	return 0;  
}

Best regards

Martin


Hello Martin,

the reason is given in the manual:
  -Wno-div-by-zero
           Do not warn about compile-time integer division by zero.
           Floating point division by zero is not warned about, as 
           it can be a legitimate way of obtaining infinities and NaNs.

So you should obtain a warning with "foo(4,0)", but not with "foo(4.,0)"

Axel


-------- Original-Nachricht --------
> Datum: Sun, 21 Jun 2009 08:34:50 -0400
> Von: John Fine <johnsfine@verizon.net>
> An: Martin Ettl <ettl.martin@gmx.de>
> CC: gcc-help@gcc.gnu.org
> Betreff: Re: why does g++ not warn about division by zero?

> Have you tried that at various optimization levels?
> 
> At lower optimization levels, the "inline" keyword should not cause the 
> function to actually get inlined.
> 
> If the function is not actually inlined, gcc cannot be aware of the 
> divide by zero.
> 
> I don't happen to know whether gcc would warn if it were aware of the 
> divide by zero, nor if actually inlining is sufficient to make it 
> aware.  I'm just saying actually inlining should be necessary to make it 
> aware of the divide by zero.
> 
> Martin Ettl wrote:
> > template <typename T1, typename T2> inline T1 foo (T1 const& a, T2
> const& b)
> > {
> > 	// ....
> >     return a/b;
> > }
> >
> > int main()
> > {
> > 	std::cout << foo(4.,0) << std::endl; 
> > 	return 0;  
> > }
> >
> > I compiled the testprogramm with following command:
> > g++ -W -Wall -pedantic -o test test.cpp
> >   

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