On 31-Jul-15 19:43, Bill Hoffman wrote:
On 7/31/2015 12:33 PM, Ruslan Baratov wrote:
> rm -rf _builds
> cmake -H. -B_builds
> cmake -H. -B_builds -DA=ON
> grep '\<B\>' _builds/CMakeCache.txt
B:STRING=There is no A
I'm not saying cache is a bad idea, I'm just saying that when users hit
such kind of situations that's one of the reason why they said CMake is
not a good script language. Just like CMake before CMP0054: user see
`if("a" STREQUAL "b")` and think "okay, 'a' is not equal to 'b' so this
condition is always false"... nope!
OK, it is a bit confusing. One solution would be to treat the cache
variables with some sort of separate look up. Basically put all cache
stuff into its own namespace.
Something like:
$CACHE{A}
Then, it would never be confused with the the variable A. However,
getting rid of the cache would not be something that could be done.
-Bill
I'm not sure I understand this solution. If you mean that user should
not mix cache/non-cache variables with the same name, I guess yes (good
idea for policy by the way). But it will not fix that kind of issues:
cmake_minimum_required(VERSION 3.0)
project(Foo)
set(A "OFF" CACHE BOOL "This is A")
if(A) # Yes, A is cache variable
set(B "Hello A" CACHE STRING "Do we use A?")
else()
set(B "There is no A" CACHE STRING "Do we use A?")
endif()
result will be quite the same. The fundamental reason of this is that
every cache (as a general term) have a context where it can be used, and
if context changing we can't reuse same cached value. For variable B
this context is value of A:
> cmake -H. -B_builds # use default value OFF
save to cache: B="There is no A" context: A=OFF
> cmake -H. -B_builds -DA=ON
load from cache: B="There is no A" context: A=OFF # that's the
problem, context is not the same, but we still using variable from cache
For more complex example let's use CMAKE_PREFIX_PATH and find_package:
find_package(Foo CONFIG REQUIRED)
message("Foo_DIR: ${Foo_DIR}")
> rm -rf _builds
> cmake -H. -B_builds -DCMAKE_PREFIX_PATH="location-A"
Foo_DIR: location-A
> cmake -H. -B_builds -DCMAKE_PREFIX_PATH="location-B"
Foo_DIR: location-A # Still location-A!
> rm -rf _builds
> cmake -H. -B_builds -DCMAKE_PREFIX_PATH="location-B"
Foo_DIR: location-B
You can said that it's kind of obvious that you must clear the cache if
you're changing such critical variables like CMAKE_PREFIX_PATH but in
this case think about tools that want to do such things automatically
and efficiently. There is no big difference between A and
CMAKE_PREFIX_PATH, in both cases it's just a CMake variable.
Ruslan
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