Associative Array versus SplObjectStorage

Question

I'm working on code to manage a collection of unique objects. The first prototype of this code utilises an associative array, basically as that's the way I've always done it.

However, I'm also keen on taking advantage of functionality that's been added to more modern versions of PHP such as [SplObjectStorage][1] for doing this instead, partly as a learning experience, partly because it's bound to offer advantages (benchmarks I've seen suggest that SplObjectStorage can be faster than arrays in a lot of cases).

The current implementation has an associative array that I check with in_array() to see if an object is already in the array before adding a new object to it.

The big problem I can see with SplObjectStorage is that it doesn't seem (at first glance) to support key/value associative array behaviour, and can only be treated as an indexed array. However, the documentation for the newer features of PHP isn't up to the standards of the documentation of more established parts of the language and I might simply be missing something.

Can I use SplObjectStorage in place of an associative array? If so, how do I define the key when adding a new object? More importantly, what are the relative advantages and disadvantages of SplObjectStorage when compared to associative arrays?

Answer 1

You shouldn't see the SplObjectStorage as a key-value store, but merely a set of objects. Something is in the set or not, but its position is not important.

The "key" of an element in the SplObjectStorage is in fact the hash of the object. It makes it that it is not possible to add multiple copies of the same object instance to an SplObjectStorage, so you don't have to check if a copy already exists before adding.

However, in PHP 5.4 there is a new method called getHash() which you can override that will return the "hash" of the object. This - in a sense - returns/set the key so you can allow it to store under different conditions.

The main advantage of SplObjectStorage is the fact that you gain lots of methods for dealing and interacting with different sets (contains(), removeAll(), removeAllExcept() etc). Its speed is marginally better, but the memory usage is worse than normal PHP arrays.

Answer 2

Results after running benchmark with 10000 iterations on PHP 5.6.13:

Type	Time to fill	Time to check	Memory
SplObjectStorage	0.021285057068	0.019490000000	2131984
Array	0.021125078201	0.020912000000	1411440

As you can see, Array is not noticeably faster than SplObjectStorage, but use 34 % less memory.

Answer 3

When all the memory allocated to array is used up, the memory allocated to it will be doubled. In this context, a collection of objects may be more effective structure.