Start recording a function for later invocation as a command:

function(<name> [arg1 [arg2 [arg3 ...]]])

Define a function named <name> that takes arguments named arg1, arg2, arg3, (...). Commands listed after function, but before the matching endfunction(), are not invoked until the function is invoked. When it is invoked, the commands recorded in the function are first modified by replacing formal parameters (${arg1}) with the arguments passed, and then invoked as normal commands. In addition to referencing the formal parameters you can reference the ARGC variable which will be set to the number of arguments passed into the function as well as ARGV0, ARGV1, ARGV2, ... which will have the actual values of the arguments passed in. This facilitates creating functions with optional arguments. Additionally ARGV holds the list of all arguments given to the function and ARGN holds the list of arguments past the last expected argument. Referencing to ARGV# arguments beyond ARGC have undefined behavior. Checking that ARGC is greater than # is the only way to ensure that ARGV# was passed to the function as an extra argument.

A function opens a new scope: see set(var PARENT_SCOPE) for details.

See the cmake_policy() command documentation for the behavior of policies inside functions.