RE: [Dev-C++] #DEFINE versus cont variables in array inicializations

[Ioannis V. <iv...@te...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of OROSZI Balazs
> Sent: Thursday, May 29, 2003 4:32 PM
> To: dev...@li...
> Subject: Re: [Dev-C++] #DEFINE versus cont variables in array 
> inicializations
> 
> 
> > 2) For theese two ways presented here, when use one and 
> when use the other?
> 
> Well, C++ encourages people to use const variables instead of #define 
> values. But I think there are circumstances, when #define is 
> better. In 
> particular, when you HIGHLY optimize for speed in an 
> algorithm, and you 
> don't want to use magic values, then it is better to use 
> #define values, 
> as the preprocessor replaces them (as if you had typed in the exact 
> value) and the compiler can make various optimizations.


A good compiler optimises out the const variable when possible (e.g. when
not accessed later in a strange way).



> For example, when you need to shift with a known value:
> 
> i = 1<<SHIFT_VAL;
> 
> This case it is better to #define SHIFT_VAL 3 for example, 
> rather than 
> const int SHIFT_VAL=3, because in the first case the compiler will 
> automatically calculate the value in compile time:
> 
> i = 1<<3 --> i = 8;
> 
> While this is not true for using const variables (I think, unless the 
> compiler is smart), and it takes more time to shift with a variable, 
> rather than with a #define. So, another example:
> int v;
> v<<SHIFT_VAL;
> Here v is a variable, and it is faster to shift it if SHIFT_VAL is a 
> #define, rather than if SHIFT_VAL is a variable. (I have tested this 
> extensively!)


Both above should be equivalent.




> 
> In your case, I don't think you need this type of low-level 
> optimisation, so I think you can safely use a const variable.


:) At the end we agree.




Ioannis Vranos
 
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