cmake检查Mac OS X，使用APPLE还是$ {APPLE}

Question

我想检查我是否在Mac OS X中，并具有以下代码

cmake_minimum_required (VERSION 3.0)
project (test)
set (FOO 1)
if (${FOO} AND ${APPLE})
  message ("MAC OS X")
endif ()

它在非OSX系统上失败，并显示错误消息

CMake Error at CMakeLists.txt:4 (if):
  if given arguments:

    "1" "AND"

  Unknown arguments specified

如果我将 ${APPLE}替换为 APPLE，则错误消失了。但是我对此有些疑惑。我们什么时候应该用 ${VAR}引用变量，什么时候不应该引用？

提前致谢。

Answer 1

简而言之:if括号内的所有内容均作为表达式求值，这就是if关键字的语义。因此，如果在其中放置APPLE，它将被评估为变量名并产生正确的结果。

现在，如果将${APPLE}放在此处，则${}将在if评估表达式之前先评估其内容。因此，就跟您写的一样

if (1 AND )

(在未设置变量 APPLE的情况下，在非OSX系统上就是这种情况)。这是无效的语法，并产生您得到的错误。您应该写:

if (FOO AND APPLE)

引用 CMake Documentation:

The if command was written very early in CMake’s history, predating the ${} variable evaluation syntax, and for convenience evaluates variables named by its arguments as shown in the above signatures. Note that normal variable evaluation with ${} applies before the if command even receives the arguments. Therefore code like:

set(var1 OFF)
set(var2 "var1")
if(${var2})

appears to the if command as:

if(var1)

and is evaluated according to the if() case documented above. The result is OFF which is false. However, if we remove the ${} from the example then the command sees:

if(var2)

which is true because var2 is defined to “var1” which is not a false constant.