Thread: I need...

I need an explanation of part of the following function that is highlighted in red (possible typo). At first I thought the parenthesis to the right of c was a typo(this is from the book Accelerated C++) but the full code compiles and runs successfully. I don't understand why that parenthesis is there.

Code:

bool not_url_char(char c)
{
	// characters, in addition to alphanumerics, that can appear in a \s-1URL\s0
	static const string url_ch = "~;/?:@=&$-_.+!*'(),";

	// see whether `c' can appear in a \s-1URL\s0 and return the negative
	return !(isalnum(c) ||
	         find(url_ch.begin(), url_ch.end(), c) != url_ch.end());
}

std::find is defined as follows:
std::find(start_it, end_it, char_to_find)
If std::find cannot find char_to_find in the range start_it to end_it, it simply returns end_it.
And the parameter you pass to end_it is url_ch.end().
Makes sense?

You also need an editor that can highlight matching parentheses. Such as, well, nearly every programmer's editor out there, with the right settings. (I like jEdit.)

Yes, the two-line return at first confused me It was the last closing parentheses that made me force myself to look at it twice.

Originally Posted by CodeMonkey

Yes, the two-line return at first confused me It was the last closing parentheses that made me force myself to look at it twice.

Which is exactly why, when I split a line of code, I put the related operator on the beginning of the next line instead of the end of the previous.

Code:

return !(isalnum(c)
    || find(url_ch.begin(), url_ch.end(), c) != url_ch.end());

Makes it pretty obvious that the second line is part of a larger expression.

Or even something like:

Code:

return 
	!(
		isalnum(c) 
		||
	        find(url_ch.begin(), url_ch.end(), c) != url_ch.end()
	);

One more thing. The canonical convention for boolean functions is the positive sense, eg:

Code:

bool url_char(char c);

In other words, negation would done by the user code, not the function itself.