CMake variable expansion using "@" vs. "${}"

Question

Consider the following:

SET(TEST_DIR, "test")
INSTALL(PROGRAMS scripts/foo.py DESTINATION ${TEST_DIR})
INSTALL(PROGRAMS scripts/foo.py DESTINATION @TEST_DIR@)

The first INSTALL command does not work. The second does. Why is that? What is the difference between those two? I have not found any reference to @@ expansion except in the context of creation of configuration files. Everything else only uses ${} expansion.

UPDATE: OK, obvious bug in the above. My SET() command has an extraneous comma. Removing it, such that it looks like:

    SET(TEST_DIR "test")

results in both @@ and ${} expansions working. Still wondering (a) what is the meaning of @@ as opposed to ${}, and why only the former worked with my incorrect SET() statement.

Answer 1

According to the documentation for the configure_file() command when configuring a file both the ${VAR} form and @VAR@ form will be replaced VAR's value. Based on your experience above and some testing I did both forms are replaced when CMake evaluates your CMakeLists.txt, too. Since this is not documented I would recommend against using the @VAR@ from in your CMakeLists.txt

Note that when using configure_file() you can restrict replacement to only the @VAR@ form by using the @ONLY argument.

Answer 2

As far as I know, the @VAR@ syntax is only used when replacing variables with the configure_file command.

Note that the configure_file command allows for an extra option @ONLY. Using it you can specify that only the @VAR@'s are replaced, but that the ${VAR}'s are kept.

As an example, this can be useful when generating e.g. a cmake-file which is later to be used with CMake again. E.g. when building your project, the @VAR@ will be replaced when using configure_file. After you distributed your project and someone else uses the generated UseProject.cmake file, the ${VAR}$ entries will be replaced.