Can I ignore the key word DEFINED in cmake? -

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i have simple question related camke keyword defined. not know in condition keyword necessary , in condition keyword can ignored. following example given illustrate question:

cmake_minimum_required(version 2.8) project(test)  if (not abc)   set(abc "hello abc") endif()  message(${abc})  if (abc)   message(${abc}) endif()  if (defined abc)   message(${abc}) endif()

as can see example, if (variable) , if (defined variable) function same. therefore, question arises: defined necessary? idea appreciated.

i notice variable may defined value can off, , in case variable still defined not on, means if(variable) false while if(defined variable) still true. 

set(initial_pass off)  if (defined initial_pass)   message(${initial_pass}) endif()  if (initial_pass)   message(${initial_pass}) endif()  set(initial_pass on)  if (defined initial_pass)   message(${initial_pass}) endif()  if (initial_pass)   message(${initial_pass}) endif()

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