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I have been studying design patterns and came accross the fly weight pattern. I have been trying to see opportunities to use the pattern in my applications but I am having trouble seeing how to use it. Also, what are some signs that a fly weight pattern is being used when I read other peoples code?

According to the definition it says:

Use sharing to support large numbers of fine-grained objects efficiently.

If I read it right Dictionaries and Hashtables could be instances of fly weights is this correct?

Thanks in advance.

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Just a little anecdote on flyweights: I once had to create large excel files (up to 500k records, over 100 columns) with a 3rd party API. The styles for the cells became extremely memory intensive. So whenever a style was needed, a hashtable was checked if an equal style already existed and then provided just a reference to this style. This modification made this export possible. Now having that much data in excel is madness in my opinion. But the controllers had their analysis macros which they wanted to keep. – Falcon Sep 15 '11 at 20:49
Thank you for these examples! I really appreciate it. – Jeremy E Sep 15 '11 at 20:51
Comment: I hope that people who write pattern and OO books and articles come to the real world of the average programmer and stop using lawyer style English! – NoChance Sep 15 '11 at 22:17
"I once had to create large excel files (up to 500k records, over 100 columns)" -- that's not much compared to what some traders are capable of creating ;-) – quant_dev Sep 15 '11 at 22:38
After reading several of these examples I would think in memory data compression would be an execellent place to implement this technique. Thanks for the help! – Jeremy E Sep 16 '11 at 13:22
up vote 13 down vote accepted

One example is in the Java libraries. Java has primitive types (e.g. int, which is a 32-bit integer) and wrappers for them (e.g. Integer, which wraps int). There are methods to "box" an int into an Integer and unbox an Integer into an int. The wrappers are necessary because the primitive types aren't objects and hence can't e.g. be used as keys in Maps or placed in Collections.

The boxing method uses an array of flyweight objects as a kind of cache for Integers corresponding to int values between -128 and 127. Since those are the values most likely to be used as keys or placed in collections, it reduces allocation and memory use. (If there are 5000000 Integers representing the value 0 floating around, that uses 5000000 times as much memory as reusing the flyweight instance).

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3… :) – Oded Sep 15 '11 at 20:44
So the intern pool for strings in C# is another example of the flyweight pattern correct? – Jeremy E Sep 15 '11 at 20:47
@Jeremy E: Yes, in my opinion you can call string interning an application of the flyweight pattern, altough for strings, it's not only about memory consumption, but also about runtime efficiency. – Falcon Sep 15 '11 at 20:57

Graphics. Typically, a raster image (which are the backbone of most consumer-level computer graphics) is CPU-cheap, but memory-expensive to work with (which is fine because memory's cheap but CPU is expensive). If that raster image is to be repeated many times in rendering a larger UI (from icons in a Windows GUI app to characters of a font in a word processor, to textures on surfaces in a 3D game), it makes a lot of sense to load the image into memory once, and simply point to it using very simple objects that are cheap to make and do not, themselves, take up a lot of memory. A sprite, which is simply a point in graphical space at which an image should be displayed, is just a 3D point and a memory pointer to the first pixel of the image to use. MAYBE it also includes the dimensions of the portion of the sprite image file to be used, either in graphical or memory terms. This information is all very inexpensive to change, say to change the image or location of the sprite, and it can be done without loading a new image each time, thus drastically increasing performance of the underlying program to manipulate and display the proper portions of the proper images to render a complete UI "scene".

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ASCII-range Character instances in Smalltalk are flyweights.

When you evaluate something like Character space, Character class >> #value: executes:

value: anInteger 
    "Answer the Character whose value is anInteger."

    anInteger > 255 ifTrue: [^self basicNew setValue: anInteger].
    ^ CharacterTable at: anInteger + 1.

The class variable CharacterTable is initialised like this:

    "Create the table of unique Characters, and DigitsValues."
    "Character initializeClassificationTable"

    CharacterTable ifNil: [
        "Initialize only once to ensure that byte characters are unique"
        CharacterTable := Array new: 256.
        1 to: 256 do: [:i | CharacterTable
            at: i
            put: (self basicNew setValue: i - 1)]].
    self initializeDigitValues

So when you create a String, the ASCII-range Characters will come from CharacterTable rather than being newly-created every time.

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Same with GUI brushes in Winforms, if you're familiar with that.

By the way, it's not only memory consumption that's problematic. Memory allocation is a relatively slow procedure.

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Could this be applied to garbage collection to reduce the amount duplication behind the scenes? I think this would go a long way to improving gc performance if it was handled in the background. – Jeremy E Sep 16 '11 at 13:25
I don't think so, because GCs will have to expect when you're going to create another object of the same kind, and reset its data instead of allocating. That saves the allocation, but it doesn't help you lose unused memory, which is the whole point behind GC. – Yam Marcovic Sep 16 '11 at 13:35

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