cmake_minimum_required(VERSION 3.1)

file(WRITE foo.cpp [[
void bar();

int main()
{
    bar();
}
]])

add_executable(foo foo.cpp)

file(WRITE bar.cpp [[
#include <iostream>

void bar()
{
    std::cout << "bar" << std::endl;
}
]])

target_sources(foo PRIVATE bar.cpp)