cmake usefulness of aliases

Question

I dont quite get the application of alias expressions. I understand that I can write something like this

cmake_minimum_required(VERSION 3.5.1 FATAL_ERROR)
project(myLibs)
add_library(${PROJECT_NAME} src/test.cpp)
add_library(myLibs::myLibs ALIAS ${PROJECT_NAME})
...

and then use

  target_link_libraries(${TARGET_NAME}
  myLibs::myLibs

in another file to link the library to some executable, etc.

but why would I do that? I might as well skip the alias definition and just use the targetname of the built library directly

target_link_libraries(${TARGET_NAME}
myLibs

• Can anyone explain to me why aliases exist
• why do they employ the :: syntax? is the alias not completely arbitrary?

Answer 1

Taking your add_library() example, the CMake target's name would e.g. directly be linked to the target's output file names.

So ALIAS targets are mainly used to give the target a more spelling or structured name by e.g. adding a "namespace".

The cmake-developer documentation gives the following advice on namespaces:

When providing imported targets, these should be namespaced (hence the Foo:: prefix); CMake will recognize that values passed to target_link_libraries() that contain :: in their name are supposed to be imported targets (rather than just library names), and will produce appropriate diagnostic messages if that target does not exist (see policy CMP0028).