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digitalmars.D.learn - simple static if / traits question...

reply WhatMeWorry <kheaser gmail.com> writes:
I'm doing conditional compilation using static ifs like so:

enum bool audio       = true;



// if audio flag is present and set to true, add to code build

static if ( (__traits(compiles, audio)) && audio)					
     playSound(soundSys, BLEEP );


This works, but I thought there might be a simpler way. For 
instance,
after perusing std.traits, I thought I would find something like
isPresent(audio) or isSymbol(audio) templates.

Or am I being obtuse here?

Thanks.
Feb 22 2017
next sibling parent Era Scarecrow <rtcvb32 yahoo.com> writes:
On Wednesday, 22 February 2017 at 21:27:47 UTC, WhatMeWorry wrote:

 I'm doing conditional compilation using static ifs like so:

 enum bool audio       = true;



 // if audio flag is present and set to true, add to code build

 static if ( (__traits(compiles, audio)) && audio)					
     playSound(soundSys, BLEEP );


  I think you're thinking too deeply on this. During optimization 
branches of if statements that always evaluate to false are 
compiled out.
Feb 22 2017
prev sibling parent reply Profile Anaysis <PA gotacha.com> writes:
On Wednesday, 22 February 2017 at 21:27:47 UTC, WhatMeWorry wrote:

 I'm doing conditional compilation using static ifs like so:

 enum bool audio       = true;



 // if audio flag is present and set to true, add to code build

 static if ( (__traits(compiles, audio)) && audio)					
     playSound(soundSys, BLEEP );


 This works, but I thought there might be a simpler way. For 
 instance,
 after perusing std.traits, I thought I would find something like
 isPresent(audio) or isSymbol(audio) templates.

 Or am I being obtuse here?

 Thanks.


You do realize that audio is a compile time constant? This means 
that in the binary, everything that depends on it is evaluated(as 
it can be, since it is known). This means that whatever app you 
are using will not be able to be able to "adapt" to the system 
changes. In this case, if the system has audio there is no way 
for the binary to use it because you compiled it out(if audio = 
false).

In such a case you do not want to use a static or compile time 
variable unless you plan on creating multiple binaries.

If your example above was just for demo then yes, you can do that 
but compiles is not what you want. Compiles only checks if the 
statement that follows is compilable as valid D code and it 
doesn't have anything to do with the value of the variables.


There are a few options:

1. static if(audio)
2. version(audio)
3. if (audio)

It looks like you are trying to create the version(audio) 
semantic(if exists then use, else don't).

Ultimately, though, if you are trying to make a binary that can 
either use audio or not depending on where it is ran, you'll have 
to stick to using actual run time variables and probably be a bit 
more organized about it.
Feb 22 2017
parent reply WhatMeForget <kheaser gmail.com> writes:
On Wednesday, 22 February 2017 at 22:37:25 UTC, Profile Anaysis 
wrote:

 On Wednesday, 22 February 2017 at 21:27:47 UTC, WhatMeWorry 
 wrote:

 I'm doing conditional compilation using static ifs like so:

 enum bool audio       = true;



 // if audio flag is present and set to true, add to code build

 static if ( (__traits(compiles, audio)) && audio)					
     playSound(soundSys, BLEEP );


 This works, but I thought there might be a simpler way. For 
 instance,
 after perusing std.traits, I thought I would find something 
 like
 isPresent(audio) or isSymbol(audio) templates.

 Or am I being obtuse here?

 Thanks.


 You do realize that audio is a compile time constant? This 
 means that in the binary, everything that depends on it is 
 evaluated(as it can be, since it is known). This means that 
 whatever app you are using will not be able to be able to 
 "adapt" to the system changes. In this case, if the system has 
 audio there is no way for the binary to use it because you 
 compiled it out(if audio = false).

 In such a case you do not want to use a static or compile time 
 variable unless you plan on creating multiple binaries.

 If your example above was just for demo then yes, you can do 
 that but compiles is not what you want. Compiles only checks if 
 the statement that follows is compilable as valid D code and it 
 doesn't have anything to do with the value of the variables.


Definitely overthought this one big time.  But there is so much 
meta goodness in std.traits that I felt compelled to use 
"compiles" :)


 There are a few options:

 1. static if(audio)
 2. version(audio)
 3. if (audio)

 It looks like you are trying to create the version(audio) 
 semantic(if exists then use, else don't).

 Ultimately, though, if you are trying to make a binary that can 
 either use audio or not depending on where it is ran, you'll 
 have to stick to using actual run time variables and probably 
 be a bit more organized about it.


option 1 is the one I was shooting for. does the static if 
(audio) just check for the existence of audio, or does it also 
check to see if audio is true as well?

I've got a game tutorial with 15 sequential projects.  Each 
project introduces a new concept by building on the previous 
project code.  But I soon had so much
code duplication that I decided to use common modules/functions. 
The specific code could be injected via flags in the apps.d     
Hence my original question.
Feb 22 2017
parent reply Profile Anaysis <PA gotacha.com> writes:
 There are a few options:

 1. static if(audio)
 2. version(audio)
 3. if (audio)

 It looks like you are trying to create the version(audio) 
 semantic(if exists then use, else don't).

 Ultimately, though, if you are trying to make a binary that 
 can either use audio or not depending on where it is ran, 
 you'll have to stick to using actual run time variables and 
 probably be a bit more organized about it.


 option 1 is the one I was shooting for. does the static if 
 (audio) just check for the existence of audio, or does it also 
 check to see if audio is true as well?


Yes, but it checks at compile time. So the code will be evaluated 
by the compiler and if audio is true, it will only compile in the 
code in the if block.

e.g,

     static if (audio) { do something }

will be identical, in the binary, to

     do something

if audio is true.

The static if is an if statement and works like any ordinary if 
statement, but since you are using static(known at compile time) 
information then static if can actually be computed/evaluated at 
compile time(since all the inputs are know and cannot be changed)
Feb 23 2017
parent Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Thursday, 23 February 2017 at 18:35:29 UTC, Profile Anaysis 
wrote:

 [...]


 option 1 is the one I was shooting for. does the static if 
 (audio) just check for the existence of audio, or does it also 
 check to see if audio is true as well?


 Yes, but it checks at compile time. So the code will be 
 evaluated by the compiler and if audio is true, it will only 
 compile in the code in the if block.

 e.g,

     static if (audio) { do something }

 will be identical, in the binary, to

     do something

 if audio is true.

 The static if is an if statement and works like any ordinary if 
 statement, but since you are using static(known at compile 
 time) information then static if can actually be 
 computed/evaluated at compile time(since all the inputs are 
 know and cannot be changed)


Or to make it simple

      static if (audio) { do somethng }

is the same thing as

      #if audio
        do something
      #endif

in C and C++.
Feb 23 2017