Since Catch2 is a single-header framework, no additional targets have to be defined and built. We only have to make sure that CMake can find catch.hpp, to build test.cpp. For convenience, we placed it in the same directory as test.cpp, but we could have chosen a different location and indicated that location by using target_include_directories. Yet another approach would be to wrap the header into an INTERFACE library. This can be done as illustrated in the Catch2 documentation (https://github.com/catchorg/Catch2/blob/master/docs/build-systems.md#cmake):

# Prepare "Catch" library for other executables
set(CATCH_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/catch)
add_library(Catch INTERFACE)
target_include_directories(Catch INTERFACE ${CATCH_INCLUDE_DIR})

We would have then linked against the library as follows:

target_link_libraries(cpp_test Catch)

We recall from the discussion in Recipe 3, Building and linking static and shared libraries, in Chapter 1, From a Simple Executable to Libraries that INTERFACE libraries are pseudo-targets offered by CMake that are useful to specify usage requirements for targets outside our project.