cmake：什么时候引用变量？ [英] cmake: when to quote variables?

问题描述

我第一次编写CMake宏，我很难理解变量如何工作。更具体地，$ {a}似乎与$ {a}具有不同的含义。

例如：

将列表传递到cmake宏

我无法理解我应该添加引号，以及更大的基本原则是什么。

解决方案

CMake的两个原则，你必须记住：

1. CMake是一个脚本语言，

$ c>与消息（$ {_ my_text}）会给予 ABC

设置（_my_list ABC）与消息（$ {_ my_list}）会给予 A; B; C

设置（_my_listABC）与消息（$ {_ my_list}）会给予 A; B; C

设置（_my_listABC）与消息（$ {_ my_list}）会给予 ABC

strong>

您应该考虑一些经验法则：

1. p>当您的变量包含文本时 - 尤其是可以包含空格或分号（并且包含路径/文件名）的文本 - 您应该添加引号。
   
   
   

   ：空格是参数之间的分隔符，分号是CMake中列表元素的分隔符。所以把引号放在一个应该是一个文本。




2. 如果您的变数包含清单，通常不要加引号。

   
   
   

   推理：如果你给一个文件列表给一个CMake命令通常期望一个字符串列表，而不是一个字符串包含一个列表。您可以看到的差异在 foreach（） 命令接受 ITEMS 或 LISTS 。




3. if（）语句是一种特殊情况，通常你甚至不用大括号。

   
   
   

   推理：字符串可以在展开后再次计算变量名称。为了防止出现这种情况，建议您只是命名要比较的内容的变量（例如 if（_my_text STREQUALABC））。





---

COMMAND 示例



• set（_my_textABC） COMMAND$ {CMAKE_COMMAND}-E echo$ {_ my_text}将
  
  
  
  
  • 调用 .exe -E echoABC
  
  
  • 给予 ABC
  
  
  
  
• 设置（_my_list ABC）与 COMMAND$ {CMAKE_COMMAND}-E echo cake> / code>


• 给予 A ， B：未找到命令， C：command not found

（_my_list ABC）与 COMMAND$ {CMAKE_COMMAND}-E echo$ {_ my_list}VERBATIM 将

• 调用 cmake.exe -E echoA; B; C


• A; B; C

ABC）与 COMMAND$ {CMAKE_COMMAND}-E echo$ {_ my_list}VERBATIM will

• 调用 cmake.exe -E echoA; B; C


• 给予 A; B; C

set（_my_listABC）与 COMMAND$ {CMAKE_COMMAND}-E echo $ {_ my_list}
VERBATIM 将

• 调用 cmake.exe -E echo ABC


• 给予 ABC

set（_my_listA + B=C）与 COMMAND$ {CMAKE_COMMAND}-E echo $ {_ my_list} VERBATIM 将

• 调用 cmake.exe -E echoA + B= C li>
  
• 给予 A + B = C

add_custom_target（） / add_custom_command c $ c> / execute_process（）





您使用 COMMAND 调用中的变量：




1. ：如上所述，它可以包含空格
   。




2. 如果您要将某些内容连接到要传递给可执行文件的单一参数，请使用引号。

   
   
   

   推理：变量可能包含参数列表，当使用引号时 - 不会正确提取（分号而不是空格）

   >
   

3. 始终使用 add_custom_target（）添加 VERBATIM 选项 / add_custom_command（）

   
   
   

   推理：否则跨平台行为未定义，

   
   $ b
   
   
   • CMake：$ {}和$ { }"
   
   
   • 设置和使用变量的CMake语法是什么？
   
   
   • 在字符串列表上循环
   
   
   • CMake与STREQUAL的空字符串比较失败
   
   
   
   

   I am writing CMake macros for the first time, and I have a hard time understanding how variable works. Most specifically, ${a} seems to have a different meaning than "${a}".

   For example here:

   Passing a list to a cmake macro

   I fail to understand when I am supposed to add quotes, and what are the bigger underlying principle.

   解决方案

   Two principles of CMake you have to keep in mind:

   1. CMake is a script language and arguments are evaluated after the variables are expanded
   2. CMake differentiates between normal strings and list variables (strings with semicolon delimiters)

   Examples

   • set(_my_text "A B C") with message("${_my_text}") would give A B C
   • set(_my_list A B C) with message("${_my_list}") would give A;B;C
   • set(_my_list "A" "B" "C") with message("${_my_list}") would give A;B;C
   • set(_my_list "A" "B" "C") with message(${_my_list}) would give ABC

   Some Rules of Thumb

   There are some rules of thumb you should consider:

   1. When your variable contains text - especially one that could contain spaces or semicolons (and that does include paths/filenames) - you should add quotes.

      Reasoning: A space is a delimiter between arguments and a semicolon is a delimiter for list elements in CMake. So put quotes around a text that is supposed to be one.

   2. If your variable contains a list you normally don't add quotes.

      Reasoning: If you give something like a file list to an CMake command it normally expect a list of strings and not one string containing a list. The difference you can see e.g. in the foreach() command accepting ITEMS or LISTS.

   3. if() statements are a special case where you normally don't even put the braces.

      Reasoning: A string could - after expansion - evaluate again to a variable name. To prevent this it's recommended to just name the variable whose content you want to compare (e.g. if (_my_text STREQUAL "A B C")).

   ---

   COMMAND Examples

   • set(_my_text "A B C") with COMMAND "${CMAKE_COMMAND}" -E echo "${_my_text}" would
     • call cmake.exe -E echo "A B C"
     • give A B C
   • set(_my_list A B C) with COMMAND "${CMAKE_COMMAND}" -E echo "${_my_list}" would
     • call cmake.exe -E echo A;B;C
     • give A, B: command not found, C: command not found
   • set(_my_list A B C) with COMMAND "${CMAKE_COMMAND}" -E echo "${_my_list}" VERBATIM would
     • call cmake.exe -E echo "A;B;C"
     • give A;B;C
   • set(_my_list "A" "B" "C") with COMMAND "${CMAKE_COMMAND}" -E echo "${_my_list}" VERBATIM would
     • call cmake.exe -E echo "A;B;C"
     • give A;B;C
   • set(_my_list "A" "B" "C") with COMMAND "${CMAKE_COMMAND}" -E echo ${_my_list} VERBATIM would
     • call cmake.exe -E echo A B C
     • give A B C
   • set(_my_list "A + B" "=" "C") with COMMAND "${CMAKE_COMMAND}" -E echo ${_my_list} VERBATIM would
     • call cmake.exe -E echo "A + B" = C
     • give A + B = C

   Some Rules of Thumb with add_custom_target()/add_custom_command()/execute_process()

   There are some rules of thumb you should consider when you use variables in COMMAND calls:

   1. Use quotes for the arguments that contain file paths (like the first argument containing the executable itself).

      Reasoning: See above, it could contain spaces

   2. Use quotes only if you want to concatenate something into a single parameter to be passed to executable that is called.

      Reasoning: A variable could contain a list of parameters which - when using quotes - won't be correctly extracted (semicolons instead of spaces)

   3. Always add the VERBATIM option with add_custom_target()/add_custom_command()

      Reasoning: Otherwise the cross-platform behavior is undefined and you could get surprises with your quoted strings.

   References

   • CMake: difference between ${} and "${}"
   • What's the CMake syntax to set and use variables?
   • Looping over a string list
   • CMake compare to empty string with STREQUAL failed

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