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CMake Generator Expressions - Conditional Expressions and Logical Operators

Popularity：263 ℃/2024-10-30 20:27:43

[Writing in front]

CMake's generator expressions are used to perform conditional judgments and logical operations at the build system level, and they are typically used in the context of target properties and generator expressions. These expressions allow you to customize the build process for different platforms, configurations, or compilers.

Links to documents cited in this article:

cmake Generator Expressions (7) - CMake 3.26.4 Documentation

[Text Begins]

This post will coverConditional Expressionsrespond in singingLogical Operators。

  • Conditional expressions:

A basic class of generator expressions relates to conditional logic. Two forms of conditional generator expressions are supported:

$<condition:true_string>

in the event thatcondition is "1", it evaluates to "true_string", if thecondition evaluates to "0", then evaluates to an empty string.condition of any other value will result in an error.

$<IF:condition,true_string,false_string>

Added in version 3.8.

in the event thatcondition is "1", it evaluates to "true_string", if thecondition is "0", it evaluates to "false_string".condition of any other value will result in an error.

Usually.condition itself is a generator expression. For example, when usingDebug When configured, the following expression expands toDEBUG_MODE, all other configurations are empty strings:

$<$<CONFIG:Debug>:DEBUG_MODE>

Example of use:

#conditional expression $<condition:true_string>
#equivalenceC++ if (condition == "1") return true_string else return ""
add_custom_command(
   TARGET MyExecutable PRE_BUILD
   COMMAND ${CMAKE_COMMAND} -E echo "[1]: WIN32: ${WIN32} | $<${WIN32}:is win32> | $<0:is 0>"
)

#conditional expression $<IF:condition,true_string,false_string>
#equivalenceC++ condition ? true_string : false_string
set(IS_DEBUG "1")
add_custom_command(
   TARGET MyExecutable PRE_BUILD
   COMMAND ${CMAKE_COMMAND} -E echo "[2]: IS_DEBUG: ${IS_DEBUG} | $<IF:${IS_DEBUG},is debug,is release>"
)

apart from1 maybe0 Boolean-like other thancondition The value can be changed by using the$<BOOL:...> Generator expressions wrap them for processing:

$<BOOL:string>

commander-in-chief (military)string convert to0 maybe1. If any of the following is true, the evaluation is "0":

  • string is empty.

  • string is the case-insensitive equivalent of0、FALSE、OFF、N、NO、IGNORE maybe NOTFOUND

  • string suffix-NOTFOUND Ending (case sensitive).

Otherwise it is calculated as "1".

When CMake variables providecondition When using the$<BOOL:...>Generator expression:

$<$<BOOL:${HAVE_SOME_FEATURE}>:-DENABLE_SOME_FEATURE>

Example of use:

#conditional expression $<BOOL:string>
#equivalenceC++ if (string) return "1" else return "0"
set(STRING2BOOL "not empty")
add_custom_command(
   TARGET MyExecutable PRE_BUILD
   COMMAND ${CMAKE_COMMAND} -E echo "[3]: STRING2BOOL: ${STRING2BOOL} | $<BOOL:${STRING2BOOL}> | $<$<BOOL:${STRING2BOOL}>:STRING2BOOL is not empty>"
)

The output of the three examples is as follows:

  • logical operator

Common Boolean logical operators are supported:

 $<AND:conditions>

included among theseconditions is a comma-separated list of Boolean expressions, all of which must be computed as1 maybe0If all the conditions are "1", the result of the whole expression is "1". If all the conditions are "1", the result of the whole expression is "1". If any condition is "0", the result of the whole expression is "0".

$<OR:conditions>

included among theseconditions is a comma-separated list of Boolean expressions. All of these must evaluate to "1" or "0". Ifcondition at least one of them is "1", then the result of the whole expression is "1". If allcondition The result of the whole expression is "0".

$<NOT:condition>

condition must be0 maybe1. Ifcondition is "1", the result of the expression is "0", otherwise it is "1".

Example of use:

#logical operator $<AND:conditions>
#equivalenceC++ if (condition && condition && ...) return "1" else return "0"
add_custom_command(
   TARGET MyExecutable PRE_BUILD
   COMMAND ${CMAKE_COMMAND} -E echo "[1]: (1 && 0 && 1) = $<AND:1,0,1>"
)

#logical operator $<OR:conditions>
#equivalenceC++ if (condition || condition || ...) return "1" else return "0"
add_custom_command(
   TARGET MyExecutable PRE_BUILD
   COMMAND ${CMAKE_COMMAND} -E echo "[2]: (1 || 0 || 0) = $<OR:1,0,0>"
)

#logical operator $<NOT:condition>
#equivalenceC++ if (!condition) return "1" else return "0"
add_custom_command(
   TARGET MyExecutable PRE_BUILD
   COMMAND ${CMAKE_COMMAND} -E echo "[3]: !1 = $<NOT:1>"
)

The results are as follows:

[Conclusion]

Link to the project (more star ah.... ⭐_⭐):

Github Address:/mengps/LearnCMake

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