Why could the contents of the .cpp file disappear after the Cmake build of the corresponding C++ project?

Question

CMake

I have written a simple program for parallel calculation of the sum of matrix elements using MPI.
After creating the Cmake file (I'm attaching it to the question) and successfully building the project using it through the terminal, the contents of the .cpp file disappeared. With what it can be connected?

project(Parallel_matrix_sum)

cmake_minimum_required(VERSION 2.8) 

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall")

add_executable(Parallel_matrix_sum Parallel_matrix_sum.cpp)

include_directories(TARGET_DIRECTORY "C:/Program Files (x86)/Microsoft SDKs/MPI/Include")

target_link_libraries(${PROJECT_NAME} TARGET_DIRECTORY "C:/Program Files (x86)/Microsoft SDKs/MPI/Include")