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(probably trivial) question about cmath library

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  (probably trivial) question about cmath library Fab <fab4100@gmail.com> - 2012-04-17 08:06 +0200
    Re: (probably trivial) question about cmath library SG <s.gesemann@gmail.com> - 2012-04-16 23:18 -0700
      Re: (probably trivial) question about cmath library Marc <marc.glisse@gmail.com> - 2012-04-17 07:00 +0000
    Re: (probably trivial) question about cmath library Juha Nieminen <nospam@thanks.invalid> - 2012-04-17 06:58 +0000
      Re: (probably trivial) question about cmath library Marc <marc.glisse@gmail.com> - 2012-04-17 07:11 +0000

#15415 — (probably trivial) question about cmath library

Dear all,

I am a little confused about the std namespace used for all c++ libs.
I am using g++ for compilation and I think my issue could also be
related to g++ rather than the c++ libs.  Here's my issue:

When I use #include <iostream> in my code, I have to use the scope
operator or global the namespace std to access e.g. cout.  However, when
I use #include <cmath> and want to use e.g. cos from that lib, the above
mentioned does somehow not apply.  Why?

Thank you,
Fab

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#15416

On Apr 17, 8:06 am, Fab wrote:
>
> When I use #include <iostream> in my code, I have to use the scope
> operator or global the namespace std to access e.g. cout.  However, when
> I use #include <cmath> and want to use e.g. cos from that lib, the above
> mentioned does somehow not apply.  Why?

std::cos is guaranteed to work, unqualified cos is not guaranteed to
work. If it works for you, chances are that cmath is implemented in
terms of the C header math.h like this:

  #ifndef STD_CMATH_INCLUDED
  #define STD_CMATH_INCLUDED

  #include <math.h>

  namespace std {
    ...
    using ::cos;
    ...
  }

  #endif

But a C++ compiler vendor might implement this differently in a way
that only introduces math functions in std namespace and NOT the
global scope (at least up to but excluding C++2011, I don't know if
this changed in C++2011).

> Thank you,
> Fab

Cheers!
SG

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#15419

SG  wrote:

> On Apr 17, 8:06 am, Fab wrote:
>>
>> When I use #include <iostream> in my code, I have to use the scope
>> operator or global the namespace std to access e.g. cout.  However, when
>> I use #include <cmath> and want to use e.g. cos from that lib, the above
>> mentioned does somehow not apply.  Why?
> 
> std::cos is guaranteed to work, unqualified cos is not guaranteed to
> work. If it works for you, chances are that cmath is implemented in
> terms of the C header math.h like this:
[...]

which most libraries have been illegally doing since the beginning.

> But a C++ compiler vendor might implement this differently in a way
> that only introduces math functions in std namespace and NOT the
> global scope

as required by C++03.

> (at least up to but excluding C++2011, I don't know if
> this changed in C++2011).

C++11 makes the almost universal implementation you described above
legal (except for a few details) but doesn't forbid the old one.

The description (basically what you said):
* math.h provides ::cos
* cmath provides std::cos
* either may provide the other, but you can't rely on it
remains the safest.

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#15418

Fab <fab4100@gmail.com> wrote:
> When I use #include <iostream> in my code, I have to use the scope
> operator or global the namespace std to access e.g. cout.  However, when
> I use #include <cmath> and want to use e.g. cos from that lib, the above
> mentioned does somehow not apply.  Why?

  What's happening with gcc in particular (and possibly with other
compilers) is that, AFAIK, <cmath> internally also includes <math.h>
(which is the C equivalent), which in turn has the math functions in
the global namespace.

  Note that if you use the unqualified math function names, you will
be calling the 'double' versions of <math.h> instead of the type-specific
versions in <cmath> (which are inside the std namespace). This can make
a difference if you are eg. using long doubles.

  (Also, using the unqualified names is not guaranteed to be portable.)

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#15420

Juha Nieminen  wrote:

> Fab <fab4100@gmail.com> wrote:
>> When I use #include <iostream> in my code, I have to use the scope
>> operator or global the namespace std to access e.g. cout.  However, when
>> I use #include <cmath> and want to use e.g. cos from that lib, the above
>> mentioned does somehow not apply.  Why?
> 
>   What's happening with gcc in particular (and possibly with other
> compilers) is that, AFAIK, <cmath> internally also includes <math.h>
> (which is the C equivalent), which in turn has the math functions in
> the global namespace.
> 
>   Note that if you use the unqualified math function names, you will
> be calling the 'double' versions of <math.h> instead of the type-specific
> versions in <cmath> (which are inside the std namespace). This can make
> a difference if you are eg. using long doubles.

Note that this is a gcc bug (a common one, so you might as well
consider it standard to be on the safe side...). math.h is also
supposed to provide all the overloads in C++ (not sure if that
requirement will still be there in C++1y).

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