Static variables in C++ Post: 302741131

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Top Forums Programming Static variables in C++ Post 302741131 by Don Cragun on Friday 7th of December 2012 12:52:51 PM

12-07-2012

Registered User

Quote:

Originally Posted by achenle

Saying code like that "depend[s] on undefined behavior" is too kind. Anyone who writes code that assumes the location of a variable based on the location of another variable is bat-!*()% crazystupid.

FWIW, multiple definitions on one line are a bad idea for other reasons, too.

What's this mean:

Code:

char* ptrA, ptrB;

Code:

char* ptrA, ptrB;

declares an object of type pointer to char named ptrA and an object of type char named ptrB. It also means that I will never hire the programmer that wrote this. Either they don't understand the rules of C or C++, or they don't understand that you shouldn't name a variable ptrx unless that variable is declared to be a pointer type (unless they're entering some kind of code obfuscation contest).

There is nothing wrong with declaring multiple objects with a single declaration (whether or not it is on one line or spread across several lines).

When writing code, use a set of style guidelines and be consistent. And if the guidelines you use aren't "common practice", make sure that anyone who will ever need to maintain your code also has a pointer to a written set of the guidelines used.

Don Cragun

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LEARN ABOUT DEBIAN

bup-margin

bup-margin(1)						      General Commands Manual						     bup-margin(1)

NAME

       bup-margin - figure out your deduplication safety margin

SYNOPSIS

       bup margin [options...]

DESCRIPTION

       bup margin  iterates  through  all  objects  in	your  bup repository, calculating the largest number of prefix bits shared between any two
       entries.  This number, n, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids.

       For example, one system that was tested had a collection of 11 million objects (70 GB), and bup margin returned 45.  That  means  a  46-bit
       hash  would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by
       its first 46 bits.

       The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects.  Since SHA-1 hashes have 160 bits,
       that  leaves 115 bits of margin.  Of course, because SHA-1 hashes are essentially random, it's theoretically possible to use many more bits
       with far fewer objects.

       If you're paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running bup margin occasionally to see	if
       you're getting dangerously close to 160 bits.

OPTIONS

       --predict
	      Guess  the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer
	      from the guess.  This is potentially useful for tuning an interpolation search algorithm.

       --ignore-midx
	      don't use .midx files, use only .idx files.  This is only really useful when used with --predict.

EXAMPLE

	      $ bup margin
	      Reading indexes: 100.00% (1612581/1612581), done.
	      40
	      40 matching prefix bits
	      1.94 bits per doubling
	      120 bits (61.86 doublings) remaining
	      4.19338e+18 times larger is possible

	      Everyone on earth could have 625878182 data sets
	      like yours, all in one repository, and we would
	      expect 1 object collision.

	      $ bup margin --predict
	      PackIdxList: using 1 index.
	      Reading indexes: 100.00% (1612581/1612581), done.
	      915 of 1612581 (0.057%)

SEE ALSO

       bup-midx(1), bup-save(1)

BUP

       Part of the bup(1) suite.

AUTHORS

       Avery Pennarun <apenwarr@gmail.com>.

Bup unknown-															     bup-margin(1)