问题

# 问题

我正在阅读[Professional CMake](https://crascit.com/professional-cmake/)，第5.5节“变量的潜在意外行为”不建议使用`$CACHE{variable}`来访问缓存变量，除非用于调试。确切的引用如下：

> 项目通常不应该使用这个，除非用于临时调试，因为它破坏了长期以来的期望，即普通变量将覆盖在缓存中设置的值。

我不理解这个理由，所以我创建了两个示例。第一个示例使用了`$CACHE{variable}`，第二个示例通过取消设置遮盖缓存变量的普通变量来避免使用它。

**方法1：**
```cmake
cmake_minimum_required(VERSION 3.27)
project(example)

set(variable "Hello" CACHE STRING "An example variable" FORCE)
set(variable "Goodbye")
message("The normal variable's value is ${variable}.")
message("The cache variable's value is $CACHE{variable}.")

方法2：

cmake_minimum_required(VERSION 3.27)
project(example)

set(variable "Hello" CACHE STRING "An example variable" FORCE)
set(variable "Goodbye")
message("The normal variable's value is ${variable}.")
unset(variable)
message("The cache variable's value is ${variable}.")

两者都产生了预期的输出：

The normal variable's value is Goodbye.
The cache variable's value is Hello.
-- Configuring done (0.0s)
-- Generating done (0.0s)
-- Build files have been written to: ...

因此，似乎$CACHE{variable}更能显示作者的意图，而${variable}虽然破坏了"长期期望"，但我很难看出有人会因此而感到困惑。

我在Google上找不到任何信息，我查看了最新版本（3.27）的CMake文档，但它没有提到是否应该使用它。

问题

目前，我更喜欢使用$CACHE{variable}的简洁性和简洁性，我想知道除了破坏"长期期望"之外，是否有任何理由不应该在CMakeLists.txt中使用$CACHE{variable}来访问所有缓存变量？

Here's the translation of your text. Let me know if you need any further modifications or the complete code:

```markdown
# 问题

我正在阅读[Professional CMake](https://crascit.com/professional-cmake/)，第5.5节“变量的潜在意外行为”不建议使用`$CACHE{variable}`来访问缓存变量，除非用于调试。确切的引用如下：

> 项目通常不应该使用这个，除非用于临时调试，因为它破坏了长期以来的期望，即普通变量将覆盖在缓存中设置的值。

我不理解这个理由，所以我创建了两个示例。第一个示例使用了`$CACHE{variable}`，第二个示例通过取消设置遮盖缓存变量的普通变量来避免使用它。

**方法1：**
```cmake
cmake_minimum_required(VERSION 3.27)
project(example)

set(variable "Hello" CACHE STRING "An example variable" FORCE)
set(variable "Goodbye")
message("The normal variable's value is ${variable}.")
message("The cache variable's value is $CACHE{variable}.")

方法2：

cmake_minimum_required(VERSION 3.27)
project(example)

set(variable "Hello" CACHE STRING "An example variable" FORCE)
set(variable "Goodbye")
message("The normal variable's value is ${variable}.")
unset(variable)
message("The cache variable's value is ${variable}.")

两者都产生了预期的输出：

The normal variable's value is Goodbye.
The cache variable's value is Hello.
-- Configuring done (0.0s)
-- Generating done (0.0s)
-- Build files have been written to: ...

因此，似乎$CACHE{variable}更能显示作者的意图，而${variable}虽然破坏了"长期期望"，但我很难看出有人会因此而感到困惑。

我在Google上找不到任何信息，我查看了最新版本（3.27）的CMake文档，但它没有提到是否应该使用它。

问题

目前，我更喜欢使用$CACHE{variable}的简洁性和简洁性，我想知道除了破坏"长期期望"之外，是否有任何理由不应该在CMakeLists.txt中使用$CACHE{variable}来访问所有缓存变量？

<details>
<summary>英文:</summary>

# Problem

I am working through [Professional CMake](https://crascit.com/professional-cmake/) and section *5.5 - Potentially Surprising Behaviour of Variables* discourages accessing cache variables using `$CACHE{variable}` except when debugging. The exact quote is:

&gt; Projects should not generally make use of this other than for temporary debugging, since it breaks the long-established expectation that normal variables will override values set in the cache.

I did not understand this reasoning so I created two examples. The first uses `$CACHE{variable}` and the second avoids it by unsetting the normal variable that is shadowing the cache variable.

**Approach 1:**
```cmake
cmake_minimum_required(VERSION 3.27)
project(example)

set(variable &quot;Hello&quot; CACHE STRING &quot;An example variable&quot; FORCE)
set(variable &quot;Goodbye&quot;)
message(&quot;The normal variable&#39;s value is ${variable}.&quot;)
message(&quot;The cache variable&#39;s value is $CACHE{variable}.&quot;)

Approach 2:

cmake_minimum_required(VERSION 3.27)
project(example)

set(variable "Hello" CACHE STRING "An example variable" FORCE)
set(variable "Goodbye")
message("The normal variable's value is ${variable}.")
unset(variable)
message("The cache variable's value is ${variable}.")

Both produce the expected output of:

The normal variable's value is Goodbye.
The cache variable's value is Hello.
-- Configuring done (0.0s)
-- Generating done (0.0s)
-- Build files have been written to: ...

It therefore seems that $CACHE{variable} is better at showing the author's intention than ${variable} and while it does break "long-established expectation", I struggle to see how someone could get confused by this.

I could not find anything on Google and I looked at the CMake documentation for the latest version (3.27) but it did not mention whether it should be used or not.

Question

At the moment, I prefer the simplicity and conciseness of $CACHE{variable} and want to know, beyond breaking "long-established expectation", if there is any reason why I should not use $CACHE{variable} to access all the cache variables in CMakeLists.txt?

答案1

得分: 1

Here's the translated code:

我只是要扩展一下273K在评论中所说的，因为虽然我同意遵循手册是“如此容易”的，但我也认为理解手册为什么建议这样做是合理的（也很重要）。

假设您正在构建一个库：

...
add_library(foolib STATIC ...)
target_compile_definitions(foolib PRIVATE "${foolib_special_behavior}")
...

其中 foolib_special_behavior 是通常在缓存中设置的变量。

现在有人试图在另一个项目中将您的库用作依赖项：

...
set(foolib_special_behavior "SPECIAL")
add_subdirectory("dependencies/foolib")
...

这是将某物包含为子目录时的常见模式：在包含子目录之前，您可以设置要使用的任何配置选项。

现在，如果您使用了 target_compile_definitions(foolib PRIVATE "$CACHE{foolib_special_behavior}")，这将不起作用 - 嵌套您的项目的项目将不得不在缓存中设置变量。

Please let me know if you need any further modifications or assistance with the code.

...
add_library(foolib STATIC ...)
target_compile_definitions(foolib PRIVATE "${foolib_special_behavior}")
...

...
set(foolib_special_behavior "SPECIAL")
add_subdirectory("dependencies/foolib")
...