[CMake] CMake terminology

[Craig Scott]

On Tue, Jun 28, 2016 at 12:04 AM, Eric Noulard <eric.noulard at gmail.com>
wrote:

>
>
> 2016-06-25 17:42 GMT+02:00 Ruslan Baratov <ruslan_baratov at yahoo.com>:
>
>> On 25-Jun-16 10:44, Craig Scott wrote:
>>
>> One of the slight wrinkles here is that the distinction between configure
>> and generation times is now a little stronger due to generator expressions.
>> In order to really understand generator expressions, you cannot really
>> avoid getting your head around configure and generate being distinct parts
>> of the process. Personally, I tend to gloss over the distinction when
>> explaining things so as not to distract the reader, but if I'm explaining
>> something to do with generator expressions, then the distinction has to be
>> made and I deliberately use the terms *configure* and *generation*
>> stages.
>>
>> I'm not sure I understand the link between "generator expressions" and
>> "confusing configure+generate stage". Do you mean "generator expressions"
>> on "configure+generate stage" vs "build stage"?
>>
>
> Generator expression came up because one needs to evaluate some variable
> value [apparently]at "build time"
> like knowing the exact name (with path) of a target (using TARGET_FILE
> generator expression in custom command).
>
> Knowing that "generator expression" are in fact evaluated during
> "generation step" and not really at build time
> is an implementation detail. (Which is a big one for implementors) but not
> really important to user.
>

My original comment was more about the fact that users have to understand
that the *result* of a generator expression is not available at configure
time. This is what makes them unique compared to things like ordinary CMake
variables. A classic case of this is where the developer may want to know
the binary directory of a particular target. In the past, they might try
using the LOCATION target property, but this would have been wrong for
multi-config generators like Xcode or Visual Studio. The right way to do it
is to use a generator expression like $<TARGET_FILE_DIR:myTarget>, but you
cannot use that in contexts where the value is needed immediately, such as
in a call to configure_file(). This example is contrived (there are other
ways to achieve the developer's goal), but it illustrates a common thought
process and if the developer doesn't understand when a generator expression
result is computed/available, they won't understand why using one in such
contexts doesn't work.

Not my best explanation, but hopefully that made sense. ;)

-- 
Craig Scott
Melbourne, Australia
http://crascit.com
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