[SOLVED] Cmake add_library ALIAS

Issue

I am trying to figure out exactly what this line is for in the cmake file of this github json project,

add_library(${NLOHMANN_JSON_TARGET_NAME} INTERFACE)
add_library(${PROJECT_NAME}::${NLOHMANN_JSON_TARGET_NAME} ALIAS ${NLOHMANN_JSON_TARGET_NAME})

Specifically with this example, what does this allow in this cmake file that otherwise would not be possible?

I see no other references to ${PROJECT_NAME}::${NLOHMANN_JSON_TARGET_NAME} in this CMakeLists.cmake, so I am confused as to what exactly this achieves.

Edit:

The key thing that this achieves, that the comment did not make obvious to me, is that it makes the targets work with the namespaces when the project is used through add_subdirectory()

Solution

This will allow using nlohmann/json project by adding it into your super project with add_subdirectory(...)

For example simple project structure:

<root project>\
     \thirdparty\json    <<-- git submodule to https://github.com/nlohmann/json
     \include\
     \src\
     CMakeLists.txt

In your project CMakeLists.txt

...
project(mySuperApp)

set(mySuperApp_SRC src/main.c)

# can under some conditions...
add_subdirectory(thirdparty/json)

add_executable(${PROJECT_NAME} ${mySuperApp_SRC})
target_link_libraries(${PROJECT_NAME} PRIVATE nlohmann_json::nlohmann_json)

Answered By - Sergei Nikulov