[Plplot-devel] Re: lack of standards for M_PI

[Andrew R. <and...@us...>]

I believe M_PI (and other maths constants) are not part of the C/C++ 
standard. (Perhaps they are in C99? I don't know.) I think they are a
BSD extension. Every compiler I have encountered has them defined 
somewhere though. 

I seem to remember that on Visual C++ you need to define 
_USE_MATH_DEFINES to get them defined. There may be a trick like this 
for MinGW/MSYS? 

I suspect the "#ifndef M_PI" route is the simplest and safest though.
Adding a examples/c++/plc++demos.h header file seems a good way to do
this. I don't feel strongly either way on whether the constant should be
PI or M_PI, but we probably should be consistent with the C version.

If there is a consensus then I don't mind doing the necessary legwork. 

Andrew

On Fri, Sep 30, 2005 at 11:12:18AM -0700, Alan Irwin wrote:
> To Andrew (Ross) and Maurice,
> 
> I am addressing this question mostly to the two of you because you
> respectively are our C++ and standards experts.
> 
> On Linux the M_PI constant is defined in /usr/include/math.h as
> 
> # define M_PI           3.14159265358979323846  /* pi */
> 
> A lot of our C++ examples use this constant.  Those examples gain access to 
> the constant by #include <cmath> which in turn on Linux includes math.h.
> However, our other Andrew has just found out that #include <cmath> does not
> define M_PI on his MinGW/MSYS system.  Furthermore, it turns out (see
> http://www.network-theory.co.uk/docs/gccintro/gccintro_27.html) that the
> M_PI constant is not an ansi standard.  In fact, that web page gives an
> example where the gcc -Wall -ansi flags choke on M_PI by design to indicate
> M_PI is not a standard.
> 
> One brute force way out of this is to insert
> 
> #ifndef M_PI
> #define M_PI 3.14159265358979323846
> #endif
> 
> into every C++ example that uses that constant, but perhaps a better option
> would be to change all the C++ examples to include
> examples/c++/plc++demos.h, a header which would be designed to handle this
> issue and all other C++ example-related includes. Note,
> examples/c/plcdemos.h plays such a role for the C examples.
> 
> Let me know what you think is the best way to go, and I will be happy
> to do the required editing.
> 
> Another related issue is the way we handle the definition of PI (where PI is
> not even defined in Linux) in c/plcdemos.h. Should I leave the definition of
> PI in plcdemos.h (in case any of our users are counting on that for their
> own code), but also define M_PI there as above and change all the C examples
> from PI to M_PI just to be consistent with the C++ examples?
> 
> Alan
> __________________________
> Alan W. Irwin
> email: ir...@be...
> phone: 250-727-2902
> 
> Astronomical research affiliation with Department of Physics and Astronomy,
> University of Victoria (astrowww.phys.uvic.ca).
> 
> Programming affiliations with the FreeEOS equation-of-state implementation
> for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
> package (plplot.org); the Yorick front-end to PLplot (yplot.sf.net); the
> Loads of Linux Links project (loll.sf.net); and the Linux Brochure Project
> (lbproject.sf.net).
> __________________________
> 
> Linux-powered Science
> __________________________
>