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When I've seen underscores used, they suggest some pattern or intended meaning by convention. For example, variables using all upper case letters indicate a constant value, such as PI. It seems that is not the case for _size.
Generally, variables beginning with an underscore are system variables, often compiler-generated. Comes from C. Usually, programmers should avoid starting a variable name with the underscore in case there's a system variable with the same name.
Give a man a fish, he'll eat for one day. Teach a man to fish, he'll drink all your beer.
Cheers, Jeff (SCJP 1.4, SCJD in progress, if you can call that progress...)
Originally posted by Bill Keller: When I've seen underscores used, they suggest some pattern or intended meaning by convention.
While it is true that there is no inherent meaning in prepending underscores -- just as there is no inherent meaning to all-caps -- there certainly is personal/group convention.
I've seen it used for the following.
Instance variable members
Constructor and setter arguments
All method arguments (only saw this once)
Personally, I dislike the practice for a couple reasons. First, the underscore is a PITA to type and easy to miss when reading code, and it looks ugly as a prefix/suffix. I did say "personally."
As well, modern IDEs can color-code member and local variables differently if you find you're getting confused. However, in that case I'd say the method is too long and/or not documented well enough.
For the majority (?) of people, using variable prefixes to denote scope and type has lost favor as OOP has gained in popularity. Nothing screams "exposed implementation details" like a variable named "mi_size".
In case we ever work together, please don't pick up this style.