The code from Chapter 1, From a Simple Executable to Libraries, Recipe 3, Building and linking shared and static libraries, is used almost unaltered: we will only add a dependency on the UUID library. This dependency is conditional, and if the UUID library is not found, we will exclude the code using it via the preprocessor. The code is properly organized into its own src subdirectory. The layout of the project is as follows:

.
├── CMakeLists.txt
├── src
│   ├── CMakeLists.txt
│   ├── hello-world.cpp
│   ├── Message.cpp
│   └── Message.hpp
└── tests
    └── CMakeLists.txt

We can already see that we have a root CMakeLists.txt with one leaf under the src subdirectory and another under the tests subdirectory.

The Message.hpp header file contains the following:

#pragma once

#include <iosfwd>
#include <string>

class Message {
public:
  Message(const std::string &m) : message_(m) {}

  friend std::ostream &operator<<(std::ostream &os, Message &obj) {
    return obj.printObject(os);
  }

private:
  std::string message_;
  std::ostream &printObject(std::ostream &os);
};

std::string getUUID();

This is the corresponding implementation in Message.cpp:

#include "Message.hpp"

#include <iostream>
#include <string>

#ifdef HAVE_UUID
#include <uuid/uuid.h>
#endif

std::ostream &Message::printObject(std::ostream &os) {
  os << "This is my very nice message: " << std::endl;
  os << message_ << std::endl;
  os << "...and here is its UUID: " << getUUID();

  return os;
}

#ifdef HAVE_UUID
std::string getUUID() {
  uuid_t uuid;

  uuid_generate(uuid);
  char uuid_str[37];
  uuid_unparse_lower(uuid, uuid_str);
  uuid_clear(uuid);
  std::string uuid_cxx(uuid_str);
  return uuid_cxx;
}
#else
std::string getUUID() { return "Ooooops, no UUID for you!"; }
#endif

Finally, the sample hello-world.cpp executable is as follows:

#include <cstdlib>
#include <iostream>

#include "Message.hpp"

int main() {
  Message say_hello("Hello, CMake World!");

  std::cout << say_hello << std::endl;

  Message say_goodbye("Goodbye, CMake World");

  std::cout << say_goodbye << std::endl;

  return EXIT_SUCCESS;
}