CURRENT: CLANG 3.3 and -stad=c++11 and -stdlib=libc++: isnan()/isninf() oddity

[David Chisnall]

On 10 Jul 2013, at 17:33, "O. Hartmann" <ohartman at zedat.fu-berlin.de> wrote:

> Hi David,
> 
> thanks for the fast response.
> 
> The code I was told to check with is this:
> 
> #include <iostream>
> #include <typeinfo>
> #include <cmath>
> 
> int
> main(void)
> {
> 
>        std::cout << typeid(isnan(1.0)).name() << "\n";
> 
> }
> 
> 
> If I compile it with 
> 
> c++ -o testme -std=c++11 -stdlib=libc++ source.cc
> 
> and run the binary, the result is "i" which I interpret as "INT".

I believe there is a bug, which is that the math.h things are being exposed but shouldn't be, however it is not the bug that you think it is.  Try this line instead:

       std::cout << typeid(std::isnan(1.0)).name() << "\n";

We have a libm function, isnan(), and a libc++ function, std::isnan().  The former is detected if you do not specify a namespace.  I am not sure what will happen if you do:

#include <iostream>
#include <typeinfo>
#include <cmath>
using namespace std;

int
main(void)
{

       cout << typeid(isnan(1.0)).name() << "\n";

}

This is considered bad form, but does happen in some code.  I am not certain what the precedence rules are in this case and so I don't know what happens.

To properly fix this, we'd need to namespace the libm functions when including math.h in C++.  This would also include fixing tweaking the macros.  

A fix for your code is to ensure isnan() and isinf() are explicitly namespaced.  Potentially, this may also work:

using std::isinf;
using std::isnan;

David

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