CMake: Howto modify a variable from inside a function

Question

What is the best practice to modify a caller's variable from inside a CMake-function. Assume

function(MyFunction IN_OUT_NAME)

   ... what to do here ...

   string(APPEND ${IN_OUT_NAME} " and that")

   ... what to do here ...

endfunction()

What needs to be done such that the following code fragment

set(MY_MESSAGE "this")
MyFunction(MY_MESSAGE)
message(${MY_MESSAGE})

delivers

this and that

Not-a-duplicate-remarks:

Modify a variable inside a function is about JavaScript not CMake
Is JavaScript a pass-by-reference or pass-by-value language? is about JavaScript not CMake

set(variable ${varaible} PARENT_SCOPE) ?

KamilCuk
– KamilCuk

2018-11-13 10:16:21 +00:00
Commented Nov 13, 2018 at 10:16 — KamilCuk
– KamilCuk, Commented Nov 13, 2018 at 10:16
Well, that sets it. But how can I read it?

Frank-Rene Schäfer
– Frank-Rene Schäfer

2018-11-13 10:16:56 +00:00
Commented Nov 13, 2018 at 10:16 — Frank-Rene Schäfer
– Frank-Rene Schäfer, Commented Nov 13, 2018 at 10:16

Inobelar · Accepted Answer · 2023-09-29 13:46:26Z

6

Just use PARENT_SCOPE to export the value to parent scope:

function(MyFunction IN_OUT_NAME)
    string(APPEND ${IN_OUT_NAME} " and that")
    set(${IN_OUT_NAME} "${${IN_OUT_NAME}}" PARENT_SCOPE)
endfunction()
    
set(MY_MESSAGE "this")
MyFunction(MY_MESSAGE)
message(${MY_MESSAGE})

Alternate way, available since CMake 3.25 - use return(PROPAGATE ...):

function(MyFunction IN_OUT_NAME)
    string(APPEND ${IN_OUT_NAME} " and that")
    return(PROPAGATE ${IN_OUT_NAME})
endfunction()

edited Sep 29, 2023 at 13:46

Inobelar

978 bronze badges

answered Nov 13, 2018 at 10:18

KamilCuk

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1 Comment

Sam Coutteau Over a year ago

Important this ( Pre 3.25, have not tested the return() ) does not work when calling the function with an variable who's name is the same as the function parameter, in this case IN_OUT_NAME. set( IN_OUT_NAME "Test2" ) +MyFunction( IN_OUT_NAME ) won't modify the variable, this can come up in functions which use each other.

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