[CMake] Functions inherit parent variables?

[Michael Hertling]

On 03/05/2012 10:43 AM, Johannes Zarl wrote:
> On Saturday 03 March 2012, 02:29:05, Robert Dailey wrote:
>> Well you're really comparing apples to oranges. C++ nested scoping rules
>> really have nothing to do with two separate functions sharing scoped
>> variables. It doesn't even really serve as a good analogy, so I can't be
>> 100% certain what you were trying to tell me ;-)
>>
>> However I appreciate your response. I really just wanted to make sure this
>> isn't a bug, because the way the called function inherits the calling
>> function's local variables is an unusual behavior for a language, at least
>> in my experience. So I had to think twice about it ;)
> 
> As Michael said: This behaviour is not at all unusual for scripting languages, 
> but there is not really a One True Way: In Tcl you have to import variables 
> explicitly, in bourne shell you overwrite values in the parent scope, ...
> It's just a property of the language that you have to know about.
> 
> So in the CMake language you should be aware that the parent scope is visible 
> inside a function, but the function does not affect the parent scope unless 
> explicitly stated:
> 
> function( test1 )
>   set( var_a "var_a inner1" )
>   message( "test1(): var_a: ${var_a}" )
> endfunction()
> 
> function( test2 )
>   set( var_a "var_a inner2" PARENT_SCOPE )
>   message( "test2(): var_a: ${var_a}" )
> endfunction()
> 
> set( var_a "var_a outer" )
> test1()
> message( "var_a: ${var_a}" )
> test2()
> message( "var_a: ${var_a}" )
> 
> --- Output:
> test1(): var_a: var_a inner1                                                                                                                                                                                                                                                   
> var_a: var_a outer                                                                                                                                                                                                                                                             
> test2(): var_a: var_a outer
> var_a: var_a inner2
> 
> Disclaimer:  Actually, this was surprising to me. I was thinking that 
> PARENT_SCOPE sets the value in the current scope plus the parent scope, not in 
> the parent scope only. I guess this could be stated in the documentation more 
> clearly...

IMO, the documentation of the PARENT_SCOPE flag is sufficiently clear:

"If PARENT_SCOPE is present, the variable will be set in the scope
*above* the current scope. Each new directory or function creates
a new scope. This command will set the value of a variable into
the *parent* directory or *calling* function (whichever is
applicable to the case at hand)."

Not a word about setting anything in the current scope, and as for
me, the explanation of CACHE/INTERNAL/FORCE, e.g., is rather vague.

Anyway, an additional remark on this thread's topic: CMake's functions
know neither return values nor pointers or references to variables, so
this kind of access from the callee to the caller's scope is the only
mechanism to transfer data from the the former to the latter, except
for properties and global variables which are often less convenient.

Regards,

Michael