# Difference between define\_property and set\_property

**URL:** <https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981>\
**Category:** Code\
**Created:** [November 18, 2024, 9:46am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981 "2024-11-18T09:46:40Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![Oodini](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/o/db5fbb/32.png) [@Oodini](https://discourse.cmake.org/u/Oodini)\
**Post date:** [November 18, 2024, 9:46am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/1 "2024-11-18T09:46:40Z")

</div>

Hello,

I am reading a CMake code with:

```auto
define_property(GLOBAL PROPERTY SomeLib_INCLUDED BRIEF_DOCS "Some doc" FULL_DOCS "Some full doc")
get_property(INCLUDED GLOBAL PROPERTY SomeLib_INCLUDED SET)

if(NOT INCLUDED)
  set_property(GLOBAL PROPERTY SomeLib_INCLUDED ON)
  ...
endif()

```

I don’t understand what is the purpose of `define_property()`. Why is it needed ?

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<div class="post-metadata">

**Author:** ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)\
**Post date:** [November 18, 2024, 10:20am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/2 "2024-11-18T10:20:51Z")

</div>

Hi.

`define_property()` is not needed in this case. If you look at [`define_property()` documentation](https://cmake.org/cmake/help/latest/command/define_property.html), you will see that in modern CMake, it’s only really necessary to use it if you want the functionality offered by `INHERITED` and/or `INITIALIZE_FROM_VARIABLE`. For properties without these, `define_property()` is unnecessary, as a property get be get and set even without being defined.

At the same time, when a project uses a custom property, it’s a good idea to document the property and its use. This can be done using normal comment blocks, or the documentation arguments of `define_property()` can be used for it. In my code, I tend to stick to plain comments and only use `define_property()` when I need the inhertiance/initialisation behaviour, but I wouldn’t object to someone using `define_property()` for it.

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<div class="post-metadata">

**Author:** ![Oodini](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/o/db5fbb/32.png) [@Oodini](https://discourse.cmake.org/u/Oodini)\
**Post date:** [November 18, 2024, 11:05am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/3 "2024-11-18T11:05:31Z")

</div>

But if `define_property()` is used, then `get_property()` always set the `INCLUDED` variable.  
So we never go in the `if()` ?

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<div class="post-metadata">

**Author:** ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)\
**Post date:** [November 18, 2024, 11:13am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/4 "2024-11-18T11:13:36Z")

</div>

> [@Oodini](#):
>
> But if `define_property()` is used, then `get_property()` always set the `INCLUDED` variable.  
> So we never go in the `if()` ?

I wasn’t able to reproduce this. When I copied the `define_property()` and `get_property()` commands from your post into a CMakeList, and then checked the `INCLUDED` variable, it was 0 (i.e. false).

Given the name of the property, is it possible that in your case, it’s set by a package/module you include?

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<div class="post-metadata">

**Author:** ![Oodini](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/o/db5fbb/32.png) [@Oodini](https://discourse.cmake.org/u/Oodini)\
**Post date:** [November 18, 2024, 11:19am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/5 "2024-11-18T11:19:38Z")

</div>

No, it is set to `ON` in the `if()`.  
So, we won’t go in this `if()` the second time we’ll include the CMake file.

But I don’t understand why we go there the first time.

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<div class="post-metadata">

**Author:** ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)\
**Post date:** [November 18, 2024, 11:25am UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/6 "2024-11-18T11:25:28Z")

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I’m confused. What first time/second time? Is this in a file included multiple times? Please bear in mind I can only reason from code you’ve shown, I know nothing about the code you _haven’t_ shown.

If this is in a file (perhaps a package config or find module) that is included multiple times, then the first time it’s read, the property will not be set, so `INCLUDED` will be false and the `if()` will be entered. This causes the property to be set (to `ON`), so next time the `get_property()` is evaluated, it will set `INCLUDED` to true and so the `if()` will be skipped. I consider this expected behaviour; what would your expactation be? Note that `define_property()` has nothing to do with this, the code shown would behave 100% the same if that command was removed.

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<div class="post-metadata">

**Author:** ![Oodini](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/o/db5fbb/32.png) [@Oodini](https://discourse.cmake.org/u/Oodini)\
**Post date:** [November 18, 2024, 12:49pm UTC](https://discourse.cmake.org/t/difference-between-define-property-and-set-property/12981/7 "2024-11-18T12:49:57Z")

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Your suppositions are good.

My initial question was due to the fact I thought `define_property()` defined _and_ set `SomeLib_INCLUDED`. So `get_property()` shoudl have set `INCLUDED`.