Return a list from the function using OUT parameter

Question

I would like to create a CMake function as:

function(test src_list dst_list)
# do something
endfunction()

usage:

test(${my_list} chg_list)

It means, my_list is a list with several fields, and chg_list will receive a list modified inside test function.

How can I create a function in CMake to do that?

Problem is you don't provide the code of the function. Is there `set(${dst_list} PARENT_SCOPE);` in it? — julp, Mar 18 '14 at 22:23

score 45 · Accepted Answer · edited Aug 30 '16 at 15:42

45

In CMake, functions have their own scope, and by default, all modification of variables are local, unless you pass CACHE or PARENT_SCOPE as parameter to set. Inside a function, if you want to modify a variable in the scope of the caller, you should use:

set(${dst_list} <something> PARENT_SCOPE)

See documentation:

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

edited Aug 30 '16 at 15:42

Antonio

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answered Mar 19 '14 at 20:58

lrineau

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score 8 · Answer 2 · answered Mar 19 '14 at 04:16

8

Check that inside your function you are set()ing not dst_list but ${dst_list}. You need this to return data to the parent scope.

answered Mar 19 '14 at 04:16

arrowd

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CiaranWelsh · Answer 3 · 2020-10-27T23:43:47.880

Here's how I solved a similar problem which was marked a duplicate of this question. This function takes a string BinaryName and adds it to the list OutVariable which is available in the parent scope. If the list is not defined it is created. I use this strategy for creating new test targets and adding the name of these targets to a list of targets in the main scope.

Thanks to @Tsyvarev for the comments that lead me to figure this out.

function(AddToListFromFunction BinaryName OutVariable)
    if ("${${OutVariable}}" STREQUAL "")
        message(STATUS "1")
        set(${OutVariable} ${BinaryName} PARENT_SCOPE)
        message(STATUS "OutVariable: ${OutVariable} ${${OutVariable}}")
    else ()
        message(STATUS "2")
        set(${OutVariable} "${${OutVariable}}" "${BinaryName}" PARENT_SCOPE)
    endif ()
endfunction()
AddToListFromFunction(MyBinary1 MyTests)
AddToListFromFunction(MyBinary2 MyTests)
message(STATUS "MyTests Variable:  ${MyTests}")

While the name of the function `AddToListFromFunction` looks self-explanatory, it would be useful to provide a little description of your "similar problem" directly in this answer. — Tsyvarev, Oct 27 '20 at 21:21

score 0 · Answer 4 · answered Jul 23 '23 at 14:36

These days, with CMake 3.25 or later, you can do this with the PROPAGATE clause of the return statement, e.g. as in:

function(test dst_list)
  set(${dst_list} "12;34")
  return(PROPAGATE ${dst_list})
endfunction()

... which is nicer to read. See also the CMake documentation for me details.

Return a list from the function using OUT parameter

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