Global static variable vs static method

[Marcin Vorbrodt]

So I have a class Math that looks like this:

Math {
public:
static Real PI(void);
};

Real Math::PI(void) {
return 4.0 * atan(1.0);
}

The problem is, that this is a class with only static methods. All the class
has are just some methods that return certain constants, it also has statics
for trig functions. A friend pointed out, that it might not be such a great
design. On the other hand, i dont want to use this:

static const Real PI = 4.0 * atan(1.0);

Because i dont know the order in which statics are initialized, and some
other static could later depend on the value of PI;

Any ideas? Is there a more graceful solution to the problem of global static
values in C++.

Martin

[Gianni Mariani]

Re: Global static variable vs static method

Marcin Vorbrodt wrote:[color=blue]
> So I have a class Math that looks like this:
>
> Math {
> public:
> static Real PI(void);
> };
>
> Real Math::PI(void) {
> return 4.0 * atan(1.0);
> }[/color]

This one would do more like what you want. It is computed on demand the
first time Math::PI is called and only the first time.

Real Math::PI(void) {
static Real pi = 4.0 * atan(1.0);

return pi;
}

[color=blue]
>
> The problem is, that this is a class with only static methods. All the class
> has are just some methods that return certain constants, it also has statics
> for trig functions. A friend pointed out, that it might not be such a great
> design. On the other hand, i dont want to use this:
>
> static const Real PI = 4.0 * atan(1.0);
>[/color]

I don't think you want static const Real PI. A static variable has to
do with a deprecated C functionality.

const Real PI = 3.14159...;

is a whole lot faster. Just compute the constants and slap them into
the text. What's wrong with that - they are constants after all !

A constant initialized with a constant expression ( no function calls -
like to atan ) is initialized at compile time. You can be guarenteed
they are set from the time the executable is loaded.


#include <cmath>

class A
{
public:

static const double pi ;

};

// initialized at compile time
const double A::pi = 3.141L;

// initialized at compile time
const double AnotherPI = 3.141L;

// initialized at compile time when executable is loaded
const double AnotherPI2 = 4 * atan( 1.0 );

[Marcin Vorbrodt]

Re: Global static variable vs static method

Thanks a bunch for responding!
Got few questions though...

"Gianni Mariani" <gi2nospam@mari ani.ws> wrote in message
news:bj96to$o6e @dispatch.conce ntric.net...[color=blue]
> Marcin Vorbrodt wrote:[color=green]
> > So I have a class Math that looks like this:
> >
> > Math {
> > public:
> > static Real PI(void);
> > };
> >
> > Real Math::PI(void) {
> > return 4.0 * atan(1.0);
> > }[/color]
>
> This one would do more like what you want. It is computed on demand the
> first time Math::PI is called and only the first time.
>
> Real Math::PI(void) {
> static Real pi = 4.0 * atan(1.0);
>
> return pi;
> }
>[/color]

Makes sense, a friend just made that sugestion to me few minutes ago
actually.
[color=blue]
>[color=green]
> >
> > The problem is, that this is a class with only static methods. All the[/color][/color]
class[color=blue][color=green]
> > has are just some methods that return certain constants, it also has[/color][/color]
statics[color=blue][color=green]
> > for trig functions. A friend pointed out, that it might not be such a[/color][/color]
great[color=blue][color=green]
> > design. On the other hand, i dont want to use this:
> >
> > static const Real PI = 4.0 * atan(1.0);
> >[/color]
>
> I don't think you want static const Real PI. A static variable has to
> do with a deprecated C functionality.
>[/color]

Deprecated C functionality. Please explain. What exactly would be a meaning
of static const variable in C++ land versus just const variable defined at
global scope.
[color=blue]
> const Real PI = 3.14159...;
>
> is a whole lot faster. Just compute the constants and slap them into
> the text. What's wrong with that - they are constants after all !
>[/color]

Real is now typedef float real. It might be double in the future. Thats why
i dont want to hardcode a value into const Real PI variable.
[color=blue]
> A constant initialized with a constant expression ( no function calls -
> like to atan ) is initialized at compile time. You can be guarenteed
> they are set from the time the executable is loaded.
>
>
> #include <cmath>
>
> class A
> {
> public:
>
> static const double pi ;
>
> };
>
> // initialized at compile time
> const double A::pi = 3.141L;
>
> // initialized at compile time
> const double AnotherPI = 3.141L;
>
> // initialized at compile time when executable is loaded
> const double AnotherPI2 = 4 * atan( 1.0 );
>[/color]

The problem with this is (I think)...
If at some later time i create another static const variable in some other
class like this:

OtherClass {
public:
static const Something = Math::PI + 3; // just for the sake of this
example :-)
};

that at the time i create Something variable, PI might not yet been
initialized.
Or does that problem not apply to primitive types?

I had this happen to me before with such code:

Vector::UNIT_X; was a static const devined in vector class, and
CoordinateSyste n::GLOBAL; was constructed using Vector::UNIT_X, Y, Z, etc

and on some compilers that would work, but on some, at the time of GLOBAL
initialization, UNIT_X vector was not yet initialized.

Thanks for you help!

Martin

[Denis Perelyubskiy]

Re: Global static variable vs static method

Marcin Vorbrodt, 9/4/2003 10:56 PM:[color=blue]
> "Gianni Mariani" <gi2nospam@mari ani.ws> wrote in message
> news:bj96to$o6e @dispatch.conce ntric.net...[color=green]
>>Marcin Vorbrodt wrote:[/color][/color]
[...snip...][color=blue][color=green]
>>I don't think you want static const Real PI. A static variable has to
>>do with a deprecated C functionality.
>>[/color]
> Deprecated C functionality. Please explain. What exactly would be a meaning
> of static const variable in C++ land versus just const variable defined at
> global scope.[/color]

I think Gianni was referring to deprecation of 'static', as used in
namespaces.

Here is a discussion: http://tinyurl.com/mbgt
There is probably more information about that in this newsgroup, as
it seems to have been discussed many times.

denis

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