“GetOrCreate” - does that idiom have an established name?

Question

Ok, consider this common idiom that most of us have used many times (I assume):

class FooBarDictionary
{
    private Dictionary<String, FooBar> fooBars;

    ...

    FooBar GetOrCreate(String key)
    {
        FooBar fooBar;

        if (!fooBars.TryGetValue(key, out fooBar))
        {
            fooBar = new FooBar();
            fooBars.Add(key, fooBar);
        }

        return fooBar;
    }
}

Does it have any kind of established name?

(Yes, it's written in C#, but it can be "easily" transferred to C++. Hence that tag.)

Answer 1

Lazy Loading

http://en.wikipedia.org/wiki/Lazy_loading

Answer 2

I always call such functions obtainSomething().

Answer 3

It sort of depends why you're doing it - the idiom is one I've seen be called memoization, caching, initialisation on demand, create on first use. Normally I call the method "ensureFoo" rather than "GetOrCreate"

Answer 4

I'm unsure of overall programming name for the high level pattern, but Perl has this wonderful behavior called Autovivification - namely, automatically creating hash (map) key with undefined value when you're querying the value of non-existing key in the hash.

Answer 5

In C#...

... I've got a DefaultingDictionary<> which does about this. As a bonus

• you can specify a default value or factory function to create values for missing keys:
• it comes with implicit conversion from IDictionary<> (wrapping the dictionary)
• it comes with extension methods to morph any dictionary into a DefaultingDictionary<> on the fly

Full Code:

• https://gist.github.com/2789882#file_defaulting_dictionary.cs

The extensions .AsDefaulting can be used to transparently use any IDictionary<> as a defaulting one, so you can opt to use any dictionary (even e.g. obtained from a thirdparty API) as a defaulting one, and the underlying container will be updated with any 'auto-vivified' items.

Use it like

IDictionary<string, MyItem> dict = LoadFromDatabase();

// using a fixed value
SomeFunc(dict.AsDefaulting(defaultItem));

// using an independent generator function
var defaulting = dict.AsDefaulting(() => new MyItem { Id = System.Guid.NewGuid() });

// using a keydepedent generator function
var defaulting = dict.AsDefaulting(key => LazyLoadFromDatabase(key));

Some test cases

are included:

• https://gist.github.com/2789882#file_defaulting_dictionary_tests.cs