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What's up with static_cast with multiple arguments???

Answer 1

+27 A:

Static cast takes one argument, but its argument is an expression, and expressions can include the comma operator. Comma is used in situations where you want to evaluate two or more expressions at once for their side effects, e.g.:

int i, j;
for (i=0, j=0; i < 10; i++,j++) {
    // do stuff
}

It's somewhat useful because without it you could only evaluate one expression each for the initializer, condition, and continue parts of the for loop (or any other place an expression is expected). It doesn't usually make for the clearest code, though, and the semantics are odd. As you observed, a comma-separated sequence evaluates to the value of its last expression.

tgamblin 2009-02-27 09:54:28

I understand the usage for it in for syntax, but I fail to grasp the functionality in a static_cast.

Statement 2009-02-27 10:06:24

There is no functionality. You just do some operations using the comma operator and cast the result.

Anonymous 2009-02-27 10:08:04

Ah, thanks to you and Ruben I now understand how it works, and learnt something new :)

Statement 2009-02-27 10:09:56

The Syntax is: static_cast<TYPE>(EXPRESSION)Expressions happen to allow commas. Note that they're not parameters; they're operators in the expression. It doesn't really make sense to do this in a static cast, it just happens to be syntactically legal.

tgamblin 2009-02-27 10:12:06

Ok. I didn't realize I could make a for loop like this : for(int i = 0; ++i, i < 10;) hehe. Not that I would go on do loops like this, but at least I feel I get the concept

Statement 2009-02-27 10:21:10

That's an evil evil FOR loop. Intelligence is knowing what you CAN do. Wisdom is knowing enough NOT to do it. Congrats on possessing both!

Mr.Ree 2009-02-27 10:37:00

This is an awesome answer.

James Thompson 2009-02-27 12:01:37

Aside from the `for` loop example, the comma operator can be useful for logging. e.g., `static_cast<int>(log("converting to int..."), 7);`

Ben Collins 2010-04-07 02:26:19

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What's up with static_cast with multiple arguments???

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