3.5 KiB
Variables and the Cache
Local Variables
We will cover variables first. A local variable is set like this:
set(MY_VARIABLE "value")
The names of variables are usually all caps, and the value follows. You access a variable by using ${}, such as ${MY_VARIABLE}.1 CMake has the concept of scope; you can access the value of the variable after you set it as long as you are in the same scope. If you leave a function or a file in a sub directory, the variable will no longer be defined. You can set a variable in the scope immediately above your current one with PARENT_SCOPE at the end.
Cache Variables
If you want to set a variable from the command line, CMake offers a variable cache. Some variables are already here, like CMAKE_BUILD_TYPE. The syntax for declaring a variable and setting it if it is not already set is:
set(MY_CACHE_VARIABLE "VALUE" CACHE STRING "Description")
This will not replace an existing value. This is so that you can set these on the command line and not have them overridden when the CMake file executes. If you want to use these variables as a make-shift global variable, then you can do:
set(MY_CACHE_VARIABLE "VALUE" CACHE STRING "" FORCE)
mark_as_advanced(MY_CACHE_VARIABLE)
The first line will cause the value to be set no matter what, and the second line will keep the variable from showing up in the list of variables if you run cmake -L .. or use a GUI.
Since BOOL is such a common variable type, you can set it more succinctly with the shortcut:
option(MY_OPTION "This is settable from the command line" OFF)
For the BOOL datatype, there are several different wordings for ON and OFF.
See cmake-variables for a listing of known variables in CMake.
The Cache
The cache is actually just a text file, CMakeCache.txt, that gets created in the build directory when you run CMake. This is how CMake remembers anything you set, so you don't have to re-list your options every time you rerun CMake.
Properties
The other way CMake stores information is in properties. This is like a variable, but it is attached to some other item, like a directory or a target. A global property can be a useful uncached global variable. Many target properties are initialized from a matching variable with CMAKE_ at the front. So setting CMAKE_CXX_STANDARD, for example, will mean that all new targets created will have CXX_STANDARD set to that when they are created. There are two
ways to set properties:
set_property(TARGET TargetName
PROPERTY CXX_STANDARD 11)
set_target_properties(TargetName PROPERTIES
CXX_STANDARD 11)
The first form is more general, and can set multiple targets/files/tests at once, and has useful options. The second is a shortcut for setting several properties on one target. And you can get targets similarly:
get_property(ResultVariable TARGET TargetName PROPERTY CXX_STANDARD)
See cmake-properties for a listing of all known properties. You can also make your own in some cases.2