Following java code returns hash code of a string.
String uri = "Some URI"
public int hashCode() {
return uri.hashCode();
}
I want to translate this code to c++. Is there any function availabe in c++ or an easy way to translate this.
-
5What do you need the hash for? This is important for the answer.Konrad Rudolph– Konrad Rudolph2011-11-11 13:49:11 +00:00Commented Nov 11, 2011 at 13:49
Add a comment
|
6 Answers
In C++03, boost::hash. In C++11, std::hash.
std::hash<std::string>()("foo");
11 Comments
user4815162342
boost::hash is not C++03, but boost. In environments that support tr1 (much more widely available than C++11), you can use std::tr1::hash<T>, defined in <tr1/functional>.
Cat Plus Plus
@user4815162342 "boost::hash is not C++03, but boost." Oh, right, how could I forget about C++Boost.
user4815162342
@jalf It was never my intention to imply that boost is a dialect of C++, just that
boost::hash is not a part of C++03 the way, for example, std::string is. I also pointed out that std::tr1::hash is an alternative available in some environments where boost isn't. It was a well-intentioned suggestion to improve an otherwise good answer, at which Cat Plus Plus unfortunately took offense.Cat Plus Plus
No, I just don't care about politics.
Stack Overflow is garbage
but
tr1::hash is not part of C++03 either. Of course, you're right that it's useful to mention both the tr1 and boost solutions, because someone might be able to use one or the other but not both. but originally you presented it as if tr1 was somehow "more C++03" than boost is, which is nonsense. :) |
Boost provides a hash function:
#include <boost/functional/hash.hpp>
int hashCode()
{
boost::hash<std::string> string_hash;
return string_hash("Hash me");
}
2 Comments
tune2fs
Its a header only library no need to build all of boost.
User12547645
Feels a bit over the top if you can just use
std::hash instead. What is the advantage of boost?The following is the source for the default String.hashCode() in Java, this is a trival exercise to implement in C++.
public int hashCode()
{
int h = hash;
if (h == 0 && count > 0)
{
int off = offset;
char val[] = value;
int len = count;
for (int i = 0; i < len; i++)
{
h = 31*h + val[off++];
}
hash = h;
}
return h;
}
2 Comments
thecoshman
well... a bit of an arse backwards way to go about it... I guess this is useful if want your hashing to be compatible with Java String.hashCode()
Bitterblue
If you initiate the values correctly with 0 and string length it delivers the same values as Java's string. But I see the "common agreement" is there is no default function to use in C/C++ ?
Personally, I like to use boost's hash functions
http://www.boost.org/doc/libs/1_47_0/doc/html/hash.html
making a string hash is pretty simple,
boost::hash<std::string> string_hash;
std::size_t h = string_hash("Hash me");
newer versions of C++ have an equivalent with std::hash
Comments
I encoutered the same question as you have, hope this code will help you :
int HashCode (const std::string &str) {
int h = 0;
for (size_t i = 0; i < str.size(); ++i)
h = h * 31 + static_cast<int>(str[i]);
return h;
}
1 Comment
Aero
The reason not to use std::hash or boost hash is we want the same return value as java.util.String.HashCode.
//For C++ Qt you can use this code, the result is the sames as for Java hashcode()
int hashCode(QString text){
int hash = 0, strlen = text.length(), i;
QChar character;
if (strlen == 0)
return hash;
for (i = 0; i < strlen; i++) {
character = text.at(i);
hash = (31 * hash) + (character.toAscii());
}
return hash;
}
2 Comments
thecoshman
Where does this random code come from? How do I know this will actually produce good quality hashes? Why on earth are suddenly assuming Qt is at play? Why are we writing code from scratch and not using a provided solution?
James
@thecoshman because sometimes you want to be able to come up with the same hashes in different languages