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:
>> 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.
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