How to write a nice function that passes variables by reference?

Question

as in said in the title, I would like to write a "nice" function in cmake that is able to modify a variable which is passed as a parameter into that function.

The only way I can think of doing it is ugly:

Function definition

function(twice varValue varName)
set(${varName} ${varValue}${varValue} PARENT_SCOPE)
endfunction(twice)

Usage

set(arg foo)
twice(${arg} arg)
message("arg = "${arg})

Result

arg = foofoo

It seems to me, there is no real concept of variables that one can pass around at all?! I feel like there is something fundamental about cmake that I didn't take in yet.

So, is there a nicer way to do this?

Thanks a lot!

github.com/boostcon/cppnow_presentations_2017/blob/master/… suggests that you use macros when you want to modify a parameter. — James Moore
– James Moore, Commented Dec 6, 2017 at 18:47
Your are correct, but it is not your fault. CMake is missing fundamental concepts of real scripting languages. — C.J.
– C.J., Commented Apr 12, 2019 at 17:23

Philipp · Accepted Answer · 2013-01-17 09:24:12Z

49

You don't need to pass the value and the name of the variable. The name is enough, because you can access the value by the name:

function(twice varName)
  SET(${varName} ${${varName}}${${varName}} PARENT_SCOPE)
endfunction()

SET(arg "foo")
twice(arg)
MESSAGE(STATUS ${arg})

outputs "foofoo"

answered Jan 17, 2013 at 9:24

Philipp

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1 Comment

nandaloo Over a year ago

thanks a lot, it's working. However, I noticed some strange behaviour, see here

Th. Thielemann · Accepted Answer · 2015-11-28 23:56:20Z

9

I use these two macros for debugging:

macro(debug msg)
    message(STATUS "DEBUG ${msg}")
endmacro()

macro(debugValue variableName)
    debug("${variableName}=\${${variableName}}")
endmacro()

Usage:

set(MyFoo "MyFoo's value")
debugValue(MyFoo)

Output:

MyFoo=MyFoo's value

edited Nov 28, 2015 at 23:56

answered Jul 22, 2014 at 10:46

Th. Thielemann

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