C++ – How ubiquitous is hash_map

ccompiler-constructionportability

The hash_map and hash_set headers aren't included in the C++ standard yet, but they're available as extensions with all the compilers I've used lately.

I'm wondering how much I can rely on these in real code without sacrificing portability. I'm working on tools projects that need to run on a host of architectures and compilers, including:

Linux (x86_64, AMD/Intel): GCC, Intel, Portland Compilers
AIX (Power): GCC, xlC
Cray XT Series (AMD): GCC, Portland, Pathscale Compilers
IBM Blue Gene Series (Power): xlC, GCC
SGI Altix (Itanium): Intel compilers
Windows: Not really a priority, but feel free to provide useful answers.

I realize some of these are pretty exotic, but that's not the point. What are your experiences with STL extensions across multiple platforms and compilers? Are they ubiquitous yet? Would you use them in your project?

Best Answer

I would probably look for the boost equivelant and use that. At least they have some pressure from their users to be platform independent. I can't imagine what would happen if you filed a bug against GCC and Intel compilers and told them to reconcile their differences on how hash_map was implemented. At best you would be able to get them to talk to each other. Suppose you even acheived that, then you've only fixed how Intel and GCC compilers are different. Good luck getting everyone else together and solving the problem within a few years.

At least with boost you know any differences across platforms are being worked out by one organization..

EDIT The boost equivalent is apparently unordered set or unordered map. (thanks Head Geek)

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

C++ – How to iterate over the words of a string

I use this to split string by a delimiter. The first puts the results in a pre-constructed vector, the second returns a new vector.

#include <string>
#include <sstream>
#include <vector>
#include <iterator>

template <typename Out>
void split(const std::string &s, char delim, Out result) {
    std::istringstream iss(s);
    std::string item;
    while (std::getline(iss, item, delim)) {
        *result++ = item;
    }
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, std::back_inserter(elems));
    return elems;
}

Note that this solution does not skip empty tokens, so the following will find 4 items, one of which is empty:

std::vector<std::string> x = split("one:two::three", ':');