Re: [Boost-build] What is _message_

Subject: Re: [Boost-build] What is _message_
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-06-03 12:08:10

On 6/3/2016 10:24 AM, aaron_at_[hidden] wrote:
>> How does the syntax:
>>
>> _message_(/check/predef//predef_check_cc_$(key))
>>
>> apply to a Boost Build variable ? I thought Boost Build variables (
>> which are list of strings ) had to use [n] to access a string in the list.
>
> The Jam language doesn't have a specific data structure for something
> like a hashmap (dicationary, associative array, etc), so it's a common
> convention to use "variable variables" to facilitate the same functionality.
>
> For example:
>
> variable-name = x ;
>
> # this sets the variable named "x"
> $(variable-name) = some list of values ;
>
> # now, read from the variable named "x"
> echo $(x) ;
>
> # outputs: some list of values
>
> The use of variable variables is often used for caching expensive
> calculations. E.g.
>
> rule my-expensive-rule ( targets * )
> {
>     # it is conventional to prefix the variable variable with
>     # a string literal so that it's easy to search for where the
>     # variable variable is being used.
>     local key = $(.my-expensive-rule-cache-$(targets)) ;
>     # this checks to see if  the variable name exists in the current
>     # module's namespace. if the variable has never been declared
>     # then the expansion evaluates to a null-like value.
>     if ! $(key)
>     {
>         # note that the variable name stored in "key" is
>         # creating a variable that is global here
>         $(key) = [ my-expensive-rule-really $(targets) ] ;
>     }
>
>     # note the double dollar signs here: $(key) returns the name
>     # of the variable and $($(key)) returns the value of the variable
>     # name stored in "key"
>     return $($(key)) ;
> }
>
> Hope that helps,

Yes it does. Thanks for illuminating the technique.