What is the use case for generator expression on target_include_directories?

Question

I've seen in multiple places references to using generator expressions when defining include directories, so you can define different places for the includes during compilation and during installation. For example:

# Define headers for this library. PUBLIC headers are used for
# compiling the library, and will be added to consumers' build
# paths.
target_include_directories(lib PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>
    PRIVATE src)

I'm building a library project and since I'm using standard paths (lib/ for the static library and include/ for the public headers), I was hoping to get away by setting CMAKE_INSTALL_PREFIX and using a simple install() call, such as:

set(CMAKE_INSTALL_PREFIX "${MY_INSTALL_DIR}")
install(TARGETS myLibrary ARCHIVE PUBLIC_HEADERS)

My expectations were that the DESTINATION would be the default for both, so I'm just telling CMake to install these kind of files. Of course it doesn't work and I need to explicitly set the destination for both libraries and header files.

So the question remains: what is the use case for the generator expressions at the beginning, if I don't seem to be able to use the INSTALL_INTERFACE anyway?

---

This is my sample CMakeLists.txt:

cmake_minimum_required(VERSION 3.12.1)
project(my_library C)

FILE(GLOB SOURCE_FILES src/*.c)
add_library(my_library ${SOURCE_FILES})
target_include_directories(my_library PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>)

set(OUTPUT_DIR "${CMAKE_BINARY_DIR}/output")
set(INSTALL_DIR "${OUTPUT_DIR}/my_library")
set(INSTALL_LIB_DIR "${INSTALL_DIR}/lib")
set(INSTALL_INC_DIR "${INSTALL_DIR}/include")
set(CMAKE_INSTALL_PREFIX "${INSTALL_DIR}")
install(TARGETS my_library ARCHIVE DESTINATION lib)
install(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/include" DESTINATION include)
# I was hoping I could do, after setting the headers with the INSTALL_INTERFACE generator expression
#install(TARGETS my_library ARCHIVE PUBLIC_HEADERS)

Answer 1

Generator-like expressions $<BUILD_INTERFACE> and $<INSTALL_INTERFACE> are used by CMake for distinguishing things, different for the build tree and for the install tree.

When building the project itself, $<BUILD_INTERFACE> is used but $<INSTALL_INTERFACE> is not.

When creating an export file with the install(EXPORT) command, things listed in $<INSTALL_INTERFACE> will be included into it, but things in $<BUILD_INTERFACE> won't.

But the creation of an export file with the export command uses $<BUILD_INTERFACE>, but does not use $<INSTALL_INTERFACE>.

Other for differentiate build and install trees expressions $<BUILD_INTERFACE> and $<INSTALL_INTERFACE> are not used.

E.g. $<INSTALL_INTERFACE> does NOT affect the install(TARGETS .. PUBLIC_HEADERS) command.