如何删除“可能无法从目标读取位置属性”？ CMake中的错误？ [英] How can I remove the "the Location property may not be read from target" error in CMake?

问题描述

我现在正在尝试在这篇文章中运行CMake脚本 。使用此脚本，CMake将打印目标的所有属性。但是，当它尝试检索目标的 LOCATION 属性时，会观察到以下错误：

I am now trying run a CMake script in this post. With this script, CMake will print all the properties of a target. However, when it tries to retrieve the LOCATION property of the target, the following error is observed:

The LOCATION property may not be read from target "abc".  Use the target
  name directly with add_custom_command, or use the generator expression
  $<TARGET_FILE>, as appropriate.

然后我尝试使用 message（$< TARGET_FILE： abc>），但这也不起作用。有任何想法吗？

Then I tried to print the property with message($<TARGET_FILE:abc>), but it does not work either. Any ideas?

推荐答案

要扩展@Florian的注释，请阅读以下内容的 LOCATION 属性在当今的CMake版本中，默认情况下，目标是错误。 CMake政策0026 的文档清楚地描述了为什么是这种情况：

To expand on @Florian's comment, reading the LOCATION property of a target is an error by default in today's CMake versions. The documentation for CMake Policy 0026 clearly describes why this is the case:

CMake 2.8.12及更低版本允许读取 LOCATION 目标属性（和特定于配置的变体）来确定构建目标的最终位置。这基于这样的假设，即在配置时所有必需的信息都可用以确定目标的最终位置和文件名。但是，直到稍后在生成时才完全确定此属性。

CMake 2.8.12 and lower allowed reading the LOCATION target property (and configuration-specific variants) to determine the eventual location of build targets. This relies on the assumption that all necessary information is available at configure-time to determine the final location and filename of the target. However, this property is not fully determined until later at generate-time.

在阅读 LOCATION之后属性，以后可以使用生成器表达式进行更改。通过将CMake设置为 OLD 行为明确：

After reading the LOCATION property, it can later be changed using generator expressions. CMake allows you to disable this policy, by setting it to the OLD behavior explicitly:

cmake_policy(SET CMP0026 OLD)

只取一点盐，因为结果可能会改变！

Just take the result with a grain of salt, as it may change!

如果您要更改政策（而不是简单地从属性列表中删除 LOCATION ），最好单独使用 OLD CMake策略。使用 OLD 行为完成后，我们可以关闭CMake策略堆栈以继续使用 NEW 行为。这是您引用的示例，已进行了修改以演示策略更改的用法：

If you're going to change the policy (instead of simply removing LOCATION from the list of properties), it is best to use OLD CMake policies in isolation. When we're done using the OLD behavior, we can POP the old policy off the CMake policy stack to resume using the NEW behavior. Here's the example you referred to, modified to demonstrate usage of the policy change:

function(echo_target tgt)
    if(NOT TARGET ${tgt})
        message("There is no target named '${tgt}'")
        return()
    endif()

    set(props
        DEBUG_OUTPUT_NAME
        DEBUG_POSTFIX
        RELEASE_OUTPUT_NAME
        ...
        LINK_SEARCH_START_STATIC
        LOCATION
        LOCATION_DEBUG
        ...
        WIN32_EXECUTABLE
        XCODE_ATTRIBUTE_WHATEVER
    )
    message(STATUS "======================== ${tgt} ========================")

    # Push the current (NEW) CMake policy onto the stack, and apply the OLD policy.
    cmake_policy(PUSH)
    cmake_policy(SET CMP0026 OLD)

    foreach(p ${props})
        # v for value, d for defined, s for set
        get_property(v TARGET ${tgt} PROPERTY ${p})
        get_property(d TARGET ${tgt} PROPERTY ${p} DEFINED)
        get_property(s TARGET ${tgt} PROPERTY ${p} SET)
        # only produce output for values that are set
        if(s)
            message(STATUS "tgt='${tgt}' p='${p}'")
            message(STATUS "  value='${v}'")
            message(STATUS "  defined='${d}'")
            message(STATUS "  set='${s}'")
            message(STATUS "")
        endif()
    endforeach()

    # Pop the previous policy from the stack to re-apply the NEW behavior.
    cmake_policy(POP)

    message(STATUS "")
endfunction()

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