Efficiently convert between Hex, Binary, and Decimal in C/C++

As others have pointed out, I would start with sscanf(), printf() and/or strtoul(). They are fast enough for most applications, and they are less likely to have bugs. I will say, however, that these functions are more generic than you might expect, as they have to deal with non-ASCII character sets, with numbers represented in any base and so forth. For some domains it is possible to beat the library functions.

So, measure first, and if the performance of these conversion is really an issue, then:

1) In some applications / domains certain numbers appear very often, for example zero, 100, 200, 19.95, may be so common that it makes sense to optimize your functions to convert such numbers with a bunch of if() statements, and then fall back to the generic library functions. 2) Use a table lookup if the most common 100 numbers, and then fall back on a library function. Remember that large tables may not fit in your cache and may require multiple indirections for shared libraries, so measure these things carefully to make sure you are not decreasing performance.

You may also want to look at boost lexical_cast functions, though in my experience the latter are relatively compared to the good old C functions.

Tough many have said it, it is worth repeating over and over: do not optimize these conversions until you have evidence that they are a problem. If you do optimize, measure your new implementation to make sure it is faster and make sure you have a ton of unit tests for your own version, because you will introduce bugs :-(

I would suggest just using sprintf and sscanf.

Also, if you're interested in how it's implemented you can take a look at the source code for glibc, the GNU C Library.

Why do these routines have to be so time-efficient? That sort of claim always makes me wonder. Are you sure the obvious conversion methods like strtol() are too slow, or that you can do better? System functions are usually pretty efficient. They are sometimes slower to support generality and error-checking, but you need to consider what to do with errors. If a bin argument has characters other than '0' and '1', what then? Abort? Propagate massive errors?

Why are you using "Dec" to represent the internal representation? Dec, Hex, and Bin should be used to refer to the string representations. There's nothing decimal about an unsigned long. Are you dealing with strings showing the number in decimal? If not, you're confusing people here and are going to confuse many more.

The transformation between binary and hex text formats can be done quickly and efficiently, with lookup tables, but anything involving decimal text format will be more complicated.

That depends on what you're optimizing for, what do you mean by "efficient"? Is it important that the conversions be fast, use little memory, little programmer time, fewer WTFs from other programmers reading the code, or what?

For readability and ease of implementation, you should at least implement both Dec2Hex() and Dec2Binary() by just calling strotul(). That makes them into one-liners, which is very efficient for at least some of the above interpretations of the word.