CMake: use of true/false or on/off or yes/no in generator expressions

Question

Is there any way to make it easy or possible to use true/false or on/off or yes/no in CMake's generator expressions, besides 1/0?

Context

As far as documentation is concerned, only 1 and 0 are recognized within generator expressions.

However, as far as the if command is concerned, 1, ON, YES, TRUE, Y are all considered boolean values.

This makes some things confusing; for example, one could use the option command to get an input from the user in the configuration phase, and then try to use its value within a generator expression; however, in this case, cmake would complain if the value is not 1 or 0. The same applies to boolean variables stored in CMake's cache.

SSCCE Example

cmake_minimum_required(VERSION 2.8.12)

add_custom_target(print
    ${CMAKE_COMMAND} -E echo $<1:hello> $<0:world>
    )

Create a CMakeLists.txt file with the above content, run cmake . and then make (generator expressions are only evaluated at build time). It will print hello, as expected.

However, the following example won't even work with the cmake command itself:

cmake_minimum_required(VERSION 2.8.12)

add_custom_target(print2
    ${CMAKE_COMMAND} -E echo $<true:hello> $<false:world>
    )

Here's its output (at configuration time):

-- Configuring done
CMake Error at CMakeLists.txt:3 (add_custom_target):
  Error evaluating generator expression:

    $<true:hello>

  Expression did not evaluate to a known generator expression


CMake Error at CMakeLists.txt:3 (add_custom_target):
  Error evaluating generator expression:

    $<false:world>

  Expression did not evaluate to a known generator expression


-- Generating done
-- Build files have been written to: /tmp/cmake

Again:

Is there any way to make it easy or possible to use true/false or on/off or yes/no in CMake's generator expressions, besides 1/0?

Or, alternatively, an easy way to convert a variable with one of these values to 1/0 so it could then be used in generator expressions?

Answer 1

This is what the $<BOOL:...> generator expression is for. See the start of the Logical Expressions section of the generator expression docs. Modifying your example:

cmake_minimum_required(VERSION 2.8.12)

add_custom_target(print2
    ${CMAKE_COMMAND} -E echo $<$<BOOL:true>:hello> $<$<BOOL:false>:world>
)