# allocPool - A simple & high performance object pool using modern C++

This is allocPool, a pool of objects in a single header file that allow you to 
avoid expensive allocations during runtime. This preallocates objects in the 
constructor (with threads) then offers you two functions: `getPtr()` and `returnPtr(ptr)`.

Using C++ concepts, we can use templates and require the class given to have a
default constructor and to have a `.reset()` function. It will be used to clean the
objects before giving them to another caller.

We avoid false sharing by keeping a high amount of work per thread. This should
lead to cache lines not being shared between threads. While this pool uses a hashmap 
and a pivot to make `returnPtr(ptr)` extremely fast, the construction's main bottleneck is
in the locking and unlocking of the hashmap's mutex. We need to do this since we cannot
write in a `std::unordered_map` at different hashes concurrently.

It will automatically grow when the max capacity is reached, though there will
be a performance penalty.

## Performance
With a simple stub class and a pool of 10000 objects, using the pool to take a pointer 
and give it back for each element is significantly faster than doing it by hand.

```
class stub {
public:
    stub() {
        for (int j{}; j < 1000; j++) { i++; }
    };
    void reset() {}

private:
    int i = 15;
};
```
```
Time (milliseconds) required for allocations without pool: 21
Time (milliseconds) required for allocations with pool: 3
Time (milliseconds) required for real allocations when constructing pool: 9
```

This trivial example shows some performance improvements that would be much more
important should the allocation and construction/destruction of the objects be more 
complex.

## Safety
AddressSanitizer, LeakSanitizer and ThreadSanitizer have been used to ensure the safety
of the class. Tests have been added to ensure the correct behavior in all cases.