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Object pool implementation in C++11

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## Object pool allocator

This is a C++11 implementation of an object pool allocator.

For more information on object pool allocators and their purpose see 
http://gameprogrammingpatterns.com/object-pool.html.

Both a fixed size pool (`FixedObjectPool`) and a dynamically growing pool
(`DynamicObjectPool`) implementation are included.

The main features of this implementation are:
* `new_object` method uses C++11 std::forward to pass construction arguments
  to the constructor of the new object being created in the pool
* `for_each` method will iterate over all live objects in the pool calling
  the given function on them
* `delete_all` method will free all pool objects at once, skipping the
  destructor call for trivial types
* maintains a freelist of next available pool entry for fast allocation

These object pool classes are not designed with exceptions in mind as most
game code avoids using exceptions.

## Example usage

```cpp
// some type to be pooled
struct Enemy {
/* some data */
Enemy(const char* name);
void update(double delta_time);
};

// created fixed size pool with space for 64 enemies
FixedObjectPool<Enemy> enemy_pool(64);

// allocate an enemy - construction parameters are forwarded
Enemy* baddie = enemy_pool.new_object("The Mekon");

// update all live enemies by executing the lambda on all allocated objects
enemy_pool.for_each([delta_time](Enemy* enemy)
    {
        enemy->update(delta_time);
    });

// delete a single enemy
enemy_pool.delete_object(baddie);

// delete all enemies at once (beware of dangling pointers)
enemy_pool.delete_all();
```

## Implementation details

Both `FixedObjectPool` and `DynamicObjectPool` are implemented using the
`ObjectPoolBlock` class.

`ObjectPoolBlock` is a single allocation containing the `ObjectPoolBlock`
instance, indices of used pool entries and the pool memory itself.

Occupancy is tracked using indexes into available entries in the block for
constant time allocation. The `ObjectPoolBlock` keeps the next free index head.
This index can be used to find the next available block entry when allocating
a new entry.

A separate list of indices is used to track occupancy versus reusing object
pool memory for this purpose to avoid polluting CPU caches with objects which
are deleted and thus no longer in use.

## Unit testing

Unit tests are written using the [Catch](https://github.com/philsquared/Catch)
unit testing framework. Unit tests are run through the `runtest` executable.

## Micro-benchmarking

This repository also includes `bench.hpp` which is a single header file
micro-benchmarking framework inspired by Catch and Rust's
[benchmarking tests](https://doc.rust-lang.org/book/benchmark-tests.html).

It's my intention to make this standalone at some point but at the
moment it's very much a work in progress.

Currently each micro-benchmark compares the performance of the following:
* Fixed pool
* Dynamic pool with 64, 128 and 256 entry blocks
* The default allocator

Benchmarks output nanoseconds per iteration (lower is better) and megabytes per
second throughput (higher is better).

## Prerequisites

The test and benchmarking applications require [CMake](http://www.cmake.org) to
generate build files.

## Compiling and running

To generate a build, compile and run follow these steps:

~~~
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make
./runtests
./runbench
~~~

## License

This software is licensed under the zlib license, see the LICENSE file for
details.

objectpool

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