[SOLVED] can't access cmake variable defined inside function using PARENT_SCOPE

Issue

So I have the following CMake code:

function(get_sources output_var)
    set(${output_var} "a" PARENT_SCOPE)
    message(${output_var}) # prints "SOURCES" (as expected)
    message(${${output_var}}) # throws an error - why?
    ...
endfunction(get_sources)

get_sources(SOURCES)
message(${SOURCES}) # prints "a" - ok, so the above function created a var

The get_sources function is supposed to create a variable with given name in parent scope and fill it with some content.

It seems like get_sources(SOURCES) creates the variable as expected - message(${SOURCES}) outside function prints "a", but there's a problem with the line message(${${output_var}}). It should evaluate to message(a) and print "a", but instead it throws an error:

CMake Error at CMakeLists.txt:41 (message):
    message called with incorrect number of arguments

I am confused. Is it supposed to be like that? Should I make a function scoped variable, fill it in and then at the end copy its contents to a parent scope variable? I checked it - it works, but I wanted it to be without any additional variables.

EDIT: I enclosed a ${${output_var}} in quotation marks and now it doesn't throw an error, but it prints nothing.

Solution

PARENT_SCOPE sets variable only for parent scope, not for the current one. This is explicitely stated in CMake documentation about 'set' command:

If the PARENT_SCOPE option is given the variable will be set in the scope above the current scope. Each new directory or function creates a new scope. This command will set the value of a variable into the parent directory or calling function (whichever is applicable to the case at hand). The previous state of the variable’s value stays the same in the current scope (e.g., if it was undefined before, it is still undefined and if it had a value, it is still that value).

This is a proper way for handle PARENT_SCOPE variables:

Should I make a function scoped variable, fill it in and then at the end copy its contents to a parent scope variable?

Answered By - Tsyvarev

Answer Checked By - Mildred Charles (WPSolving Admin)