02-cmake语法-if

格式：

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

　　

例子：

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 是 CMAKE 的关键字，用于字符串比较，相同返回 true

${CMAKE_SOURCE_DIR} 是 CMAKE 的自保留变量（拿来用就可以，含义已经确定），文件路径

${CMAKE_BINARY_DIR}是输出路径