In this recipe, we will create a simple implementation of an object pool and learn how to use it in your applications:

1. In your working ~/test directory, create a subdirectory called objpool.
2. Use your favorite text editor to create a  objpool.cpp file in the objpool subdirectory. Let's define a templated ObjectPool class. We start with the private data members and a constructor:

#include <iostream>

template<class T, size_t N>
class ObjectPool {
  private:
    T objects[N];
    size_t available[N];
    size_t top = 0;
  public:
    ObjectPool(): top(0) {
      for (size_t i = 0; i < N; i++) {
        available[i] = i;
      }
    }

3. Now let's add a method to get elements from the pool:

    T& get() {
      if (top < N) {
        size_t idx = available[top++];
        return objects[idx];
      } else {
        throw std::runtime_error("All objects are in use");
      }
    }

4. Next, we add a method that returns an element to the pool:

    void free(const T& obj) {
      const T* ptr = &obj;
      size_t idx = (ptr - objects) / sizeof(T);
      if (idx < N) {
        if (top) {
          top--;
          available[top] = idx;
        } else {
          throw std::runtime_error("Some object was freed more than once");
        }
      } else {
        throw std::runtime_error("Freeing object that does not belong to
       the pool");
      }
     }

5. Then, wrap up the class definition with a small function that returns the number of elements that are requested from the pool:

    size_t requested() const { return top; }
    };

6. Define a data type to be stored in the object pool as shown in the following code:

struct Point {
  int x, y;
};

7. Then add code that works with the object pool:

int main() {
  ObjectPool<Point, 10> points;

  Point& a = points.get();
  a.x = 10; a.y=20;
  std::cout << "Point a (" << a.x << ", " << a.y << ") initialized, requested "        <<
    points.requested() << std::endl;

  Point& b = points.get();
  std::cout << "Point b (" << b.x << ", " << b.y << ") not initialized, requested " <<
    points.requested() << std::endl;

  points.free(a);
  std::cout << "Point a(" << a.x << ", " << a.y << ") returned, requested " <<
    points.requested() << std::endl;

  Point& c = points.get();
  std::cout << "Point c(" << c.x << ", " << c.y << ") not intialized, requested " <<
    points.requested() << std::endl;

  Point local;
  try {
    points.free(local);
  } catch (std::runtime_error e) {
    std::cout << "Exception caught: " << e.what() << std::endl;
  }
  }

8. Create a file called CMakeLists.txt in the loop subdirectory with the following content:

cmake_minimum_required(VERSION 3.5.1)
project(objpool)
add_executable(objpool objpool.cpp)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

SET(CMAKE_CXX_FLAGS "--std=c++11")
set(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabi-g++)

9. Build the application and copy the resulting executable binary to the target system. Use recipes from Chapter 2,Setting Up the Environment, to do it.
10. Switch to the target system terminal. Log in using user credentials, if needed.
11. Run the binary.