02-cmake syntax-if

Format:

if(expression)
  # then section.
  COMMAND1(ARGS ...)
  COMMAND2(ARGS ...)
  ...
elseif(expression2)
  # elseif section.
  COMMAND1(ARGS ...)
  COMMAND2(ARGS ...)
  ...
else()
  # else section.
  COMMAND1(ARGS ...)
  COMMAND2(ARGS ...)
  ...
endif()

　　

example:

if(" ${CMAKE_SOURCE_DIR}" STREQUAL " ${CMAKE_BINARY_DIR}")
  message(FATAL_ERROR "
FATAL: In-source builds are not allowed.
       You should create a separate directory for build files.
")
endif()

　　

STREQUAL is the keyword of CMAKE, used for string comparison, the same returns true

${CMAKE_SOURCE_DIR}  is a self-reserved variable of CMAKE (you can use it, the meaning has been determined), the file path

${CMAKE_BINARY_DIR} is the output path