Ternary operator in CMake's generator expressions

Note that cmake 3.8 added exactly what you want to generator expressions ...

$<IF:?,true-value...,false-value...>
true-value... if ? is 1, false-value... if ? is 0

Example usage:

target_link_libraries(MyLib PUBLIC
    $<IF:$<CONFIG:Debug>,cppzmq,cppzmq-static>
    )

Where cppzmq is shared library used in Debug build and cppzmq-static is static library used in other case e.g. Release

Here's a working example, with a macro:

cmake_minimum_required(VERSION 2.8.12)

macro(ternary var boolean value1 value2)
    set(${var} $<${${boolean}}:${value1}>$<$<NOT:${${boolean}}>:${value2}>)
endmacro()

set(mybool 0)
ternary(myvar mybool hello world)

add_custom_target(print
    ${CMAKE_COMMAND} -E echo ${myvar}
    )

Create a CMakeLists.txt file and run cmake . && make print (generator expressions are only evaluated at build time).

Try changing the value of mybool to 0 or 1 and see what happens.

The following definition also works, and it is clearer:

cmake_minimum_required(VERSION 2.8.12)

macro(ternary var boolean value1 value2)
    if(${boolean})
        set(${var} ${value1})
    else()
        set(${var} ${value2})
    endif()
endmacro()

set(mybool 0)
ternary(myvar mybool hello world)

add_custom_target(print
    ${CMAKE_COMMAND} -E echo ${myvar}
    )

TL;DR

ternary(var boolean value1 value2)

means, comparing to C/C++:

int var = boolean ? value1 : value2;