"set" syntax in CMake

Question

I am trying to learn CMake from http://www.cmake.org/cmake-tutorial/. I don't follow how set syntax works. For example, from this tutorial,

set (EXTRA_LIBS ${EXTRA_LIBS} MathFunctions)

I could understand that here we want to assign MathFunctions to the EXTRA_LIBS variable. But what I am confusing is why we want to have EXTRA_LIBS ${EXTRA_LIBS}. Why not just

set (EXTRA_LIBS MathFunctions)

Moreover, I test with following code

set (VALUE_1 "value 1")  # A
set (VALUE_2 ${VALUE_2} "value 2")  # B

message("value 1:" ${VALUE_1})
message("value 2:" ${VALUE_2})

in this case, both # A and # B produce same format of result.

So my question is what is difference between # A and # B?

It's so that EXTRA_LIBS will be set to its current value and MathFunctions appended. E.g. if EXTRA_LIBS is foo bar then set (EXTRA_LIBS ${EXTRA_LIBS} MathFunctions) expands to set (EXTRA_LIBS foo bar MathFunctions) thus setting EXTRA_LIBS to foo bar MathFunctions. — Biffen
– Biffen, Commented Mar 16, 2015 at 13:59

Gluttton · Accepted Answer · 2015-03-16 14:44:30Z

So my question is what is difference between # A and # B

In first case #A you assign "value 1" to variable VALUE_1, but in second case you assign to variable VALUE_2 already existed value of this variable which is concatenated with "value 2".

For example:

set (VALUE_1 "default value")
set (VALUE_2 "default value")

set (VALUE_1 "value 1")  # A
set (VALUE_2 ${VALUE_2} "value 2")  # B

message("value 1:" ${VALUE_1})
message("value 2:" ${VALUE_2})

Output:

...
value 1:value 1
value 2:default valuevalue 2
...

Another words the second case is the way to modify already existed variable.

It can be useful when you don't want overwrite existed value, for instance:

MainProject/CMakeLists.txt:

set (CXX_COMPILER_FLAG "some optimization flags")
add_subdirectory (SubProject)

MainProj/SubProject/CMakeLists.txt:

set (CXX_COMPILER_FLAG "some warning flags") #<- wrong, overwrite flags.
set (CXX_COMPILER_FLAG "${CXX_COMPILER_FLAG} some warning flags") #<- correct, safe existed flags and add new flags.

Gray · Accepted Answer · 2019-02-28 12:16:57Z

0

set (var_a "value a") # just like: var_a = "value a"
set (var_b ${var_b} "value b") # just like: var_b = var_b + "value b"

answered Feb 28, 2019 at 12:16

Gray

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