one thing java generics are missing

phrakture · 2005-03-23 23:16:11

specialization!

I'll give you a templated example with a function here:

template <typename T>
inline std::string to_string(const T& t)
{
    std::istringstream ss;
    ss << t;
    return ss.str();
}

that's one of those standard conversion snippets that everyone uses... as long as T supports the "<<" operator, it will convert just fine...

Now, if you're using this function in other template based code, the problem comes up:

template <typename T>
void foo(const T& t)
{
    std::string s = to_string(t);
    ... do something with s now...
}

it makes sense, if you want to do operations with a string... but what if the type T *was* a string... the to_string function just did a whole mess of needless operations (stringstream conversion)...
here comes template specialization to the rescue:

template <> inline std::string to_string<std::string>(const std::string& t)
{
    return t;
}

there we go... no extra steps... because it's inlined to begin with, the compiler will remove it... (this is what's called a conversion shim). Now when the to_string function is called with an std::string argument, it does nothing.

Dusty · 2005-03-23 23:26:25

I'm not absolutely sure I understand the way the last code snippet works, but I think you're right.

Although, I *assume* javac would notice that s.toString() just returns 'this' and would optimize accordingly by itself. However, that's just a this instance counter example, and not generally applicable.

phrakture · 2005-03-23 23:32:44

Hmmm, it seems I didn't explain it too well:

the 3rd snippit is the same as the first, however the "typename T" has been removed and all instances of "T" have been replaced with a specific type: this is template specialization... it allows for you to have different code for each type... it's a complicated form of function overloading....

what I'm saying is that, in a java generic collection, you may have all this code that works for all types, but could be optimized for one of those types... I used string above...

for instance... let's say there's a member function to count the number of characters used if the type were converted to a string... the easiest thing to do would be to convert it to a string an get the length.... however, in the case of numbers, there's a much, much more efficient way to do it (can't recall the algorithm... but you can get the number of digits with division and maybe a log10 or something...)... this is a mundane case, but even if you did want to use that math for the int version of that generic, you couldn't...

phrakture · 2005-03-23 23:36:35

int digits = 1 + int(log(x)/log(10));

yeah it's 1 + log10(x)... but of course there's no log10 in the c stdlib...

Dusty · 2005-03-24 00:30:06

Ok, now I get it. I think someday I'm going to have to brush up on my C++. I'm ok reading C, but C++ extras aren't too familiar.

Dusty

Arch Linux

#1 2005-03-23 23:16:11

one thing java generics are missing

#2 2005-03-23 23:26:25

Re: one thing java generics are missing

#3 2005-03-23 23:32:44

Re: one thing java generics are missing

#4 2005-03-23 23:36:35

Re: one thing java generics are missing

#5 2005-03-24 00:30:06

Re: one thing java generics are missing

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