[cmake-developers] Exclusive Or operator

[Robert Dailey]

CMake tends to be incredibly verbose, so I like to avoid code like
you've just shown me.

Sometimes I have features that require two cache variables. Either
both must be defined, or neither (in the latter case, the feature is
disabled). Specifying one or the other is erroneous. A simple test
would be like this with XOR:

if( var1 AND var2 )
    message( "Feature is ON" )
elseif( var1 XOR var2 )
    message( SEND_ERROR "Both variables must be specified" )
endif()

However, because I do not have XOR, I have to do crazy logic:

if( var1 AND var2 )
    message( "Feature is ON" )
elseif( (NOT var1 AND var2) OR (var1 AND NOT var2) )
    message( SEND_ERROR "Both variables must be specified" )
endif()

On Wed, Sep 12, 2012 at 11:30 AM, David Cole <david.cole at kitware.com> wrote:
> Do you have a concrete example of why an IF XOR construct would be
> useful to you? (i.e. -- do you encounter the need for this logical
> construct frequently in your CMakeLists files?)
>
> It would certainly be easy to have a macro/function in the CMake
> language that returns the logical equivalent of an XOR in a variable,
> and then you could do a simple if test on that variable immediately
> after calling that function...
>
> That requires no changes to CMake, but would not be quite as readable.
>
> And just like python, if you only care about the boolean state of a
> variable, you could always do the "casting" something like this:
>
> if(var1)
>   set(var1 1)
> else()
>   set(var1 0)
> endif()
> # same for var2, then
> if(NOT ${var1} EQUAL ${var2})
>   # voila, one is true, the other is not
> else()
>   # they're both the same
> endif()
>
>
> On Wed, Sep 12, 2012 at 12:00 PM, Robert Dailey
> <rcdailey.lists at gmail.com> wrote:
>> Honestly I'm not really worried about the "band wagon" or what other
>> languages do. CMake is its own independent entity and domain language,
>> if the feature is useful then it should have it :) Anyway I was just
>> wondering if the feature would be accepted & useful. As I said I don't
>> mind doing the work.
>>
>> Not every operator needs to short circuit. If you can't short circuit,
>> then you can't. For example, in C++ you understand what short
>> circuiting rules are and you work with that. But not every C++
>> operator allows it.
>>
>> Also FWIW, in Python you can kind-of get XOR by converting both
>> operands to boolean:
>>
>> bool(a) != bool(b)
>>
>> But we can't cast operands to bool on-the-fly in CMake (AFAIK) so this
>> wouldn't be feasible.
>>
>> On Wed, Sep 12, 2012 at 6:49 AM, Brad King <brad.king at kitware.com> wrote:
>>> On 09/11/2012 05:09 PM, Robert Dailey wrote:
>>>> I don't see an exlusive or operator in CMake. Could I add one? Would
>>>> such a feature be accepted?
>>>>
>>>> Syntax would be:
>>>>
>>>> if( var1 XOR var2 )
>>>
>>> Many languages like C, C++, python, lua, etc. do not offer
>>> logical XOR operators, only bitwise XOR.  Unlike AND/OR/NOT
>>> the XOR operation cannot be used in short-circuit evaluation
>>> (CMake's if() command does not currently short-circuit, but
>>> it could.).
>>>
>>> -Brad
>> --
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