C++ unsigned long and <

Question

I have the following code:

while( int()(uStartFrame - iFrameOffset) < 1)
{
    iFrameOffset--;
}

uStartFrame and iFrameOffset are both unsigned long, so the < statement is a little difficult, I think. However, I thought that I fixed it using int(). But the loop runs infinitively, so I guess it doesn't work.

Can somebody help?

Thank you!

Why do you cast (should be (int) by that way) them? Since they are the same type it does not matter. — Araw, Jan 08 '14 at 12:56
What is this loop intended to achieve? (I'm guessing that `iFrameOffset` is initialy greater than `uStartFrame`?) — jrok, Jan 08 '14 at 13:03

score 2 · Accepted Answer · answered Jan 08 '14 at 12:55

2

while( uStartFrame < iFrameOffset + 1)
{
    iFrameOffset--;
}

Or even better

if(uStartFrame < iFrameOffset + 1)
    iFrameOffset = uStartFrame - 1;

The last line also shows the possible error. If uStartFrame is 0, then there's no unsigned long variable x that can fulfil uStartFrame == x + 1.

answered Jan 08 '14 at 12:55

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I think you misread my code. I am querying if (uStartFrame-iFrameOffset) <1 not (uStartFrame < iFrameOffset) <1 – tmighty Jan 08 '14 at 12:58
1

`a - b < 1` is true if and only if `a < b + 1` is true. That's basic math. – Zeta Jan 08 '14 at 12:59
@zeta: or even `iFrameOffset = std::min(iFrameOffset, uStartFrame - 1);` – Jarod42 Jan 08 '14 at 13:10

mockinterface · Answer 2 · 2014-01-08T13:04:06.000

The cast is wrong, you should cast it like this,

(int)(uStartFrame - iFrameOffset) < 1

but this C-style cast is not really C++ style, in your case a static_cast is preferable:

static_cast<int>(uStartFrame - iFrameOffset) < 1
static_cast<unsigned long>(uStartFrame - iFrameOffset) < 1

Apart from that, when you write int()(x) you define a function that returns an integer and accepts no parameters, and then invoke it with uStartFrame - iFrameOffset as an argument. It shouldn't even compile, well at least gcc 4.8 rightfully complains about this.

Your compiler obviously does compile it and maybe even wrongly treats it as a function that returns an un-initialized integer, most likely 0, and that possible explains why your loop runs forever.

score 0 · Answer 3 · answered Jan 08 '14 at 12:59

You're casting the test (uStartFrame-iFrameOffset) into int, not the iFrameOffset. So, if the iFrameOffset is big (max could be 2^64-1 - or bigger depending on system), then you might need 2^64 loops to get to the end.

This could be as much as a giga seconds. So, you should rethink this loop. It's not a goo idea.

C++ unsigned long and <

3 Answers3