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Is there any difference between the following two?

set(FOO true CACHE BOOL "description")

option(FOO "description" ON)

Documentation: set - option

Background: Even if I have been using CMake for a while, I only noticed the option command today and I have therefore always been using set: I was wondering if it is safe/worth to replace the first with the second.

Antonio
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2 Answers2

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In your example, there is no difference. But there can be differences:

  • option has a default value of OFF.
  • You can make an option dependent on other options with CMakeDependentOption
  • set accepts types other than BOOL, especially ;-lists.

Additionally, when you use an option you are indicating to your user that it can be set safely. Setting internal variables from the outside might subtly break the script with invalid values.

starball
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usr1234567
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  • Regarding your last point, when using `set`, by making the variable a boolean don't I prevent the user from giving it a different kind of value e.g. in the command line? Anyway, I find your second point pretty relevant. – Antonio Apr 01 '16 at 14:52
  • Frankly, I haven't ever tried that. But I think I remember CMake GUI adjusts its input method depending on the variable type. – usr1234567 Apr 01 '16 at 14:54
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    That I can confirm (for a boolean variable it gives you a tick box) – Antonio Apr 01 '16 at 14:54
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    However, if you use the command line -D approach, rather than ccmake, CMake is perfectly happy to let you set a BOOL variable/option to non-BOOL values whether you use `option` or `set(...CACHE BOOL)`. So it seems this "type safety" only applies for ccmake/cmake-gui. – kdopen Feb 08 '18 at 11:10
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    Regarding the first point: `option` accepts an optional initial value. If no initial value is provided, `OFF` is used. – Cnly May 28 '21 at 05:31
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Stumbled on this question, and thought I added an update.

As explained here, the option command does not create a CACHE variable if a normal variable with the same name exists. This behavior was introduced in version 3.13 (where it is the default). On the other hand, setting a CACHE variable named FOO when FOO exists as a normal variable, will yield two copies of FOO, a normal variable and a CACHE one.

Therefore, whether to use option(FOO "" ON) or set(FOO ON CACHE BOOL "") may also depend on what you want your configuration system to do when a variable with the same name has already been defined upstream.

Note: the two still behave the same way if FOO has been defined upstream in the cache. The difference is only if the upstream is a normal variable.

bartgol
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