The capability to inline a function call is both a compiler-specific optimization and a common behavior. That is, many compilers can do it, but they aren't required to.
Rob Kennedy
2009-09-08 02:46:42
+2
A:
The desired optimisation doesn't (just) require inlining. It requires that the length of the string be computed at compile time.
Steve Jessop
2009-09-08 10:05:20
That's not really an optimization, though. The length wouldn't be computed at run time and still call any `_lenof` function. Doesn't the standard *require* implementations to give string literals the type `const char[N]`? And aren't values of such type required to make the compiler deduce the arguments of the template function to be `N`?
Rob Kennedy
2009-09-08 13:53:22
Sorry, I misunderstood what your answer was referring to - for some reason I though you were talking about "not efficient because of [the strlen] function call". If a compiler can't inline _lenof, then it probably can't inline anything, and would be a pretty poor C++ compiler in general. Any serious use of templates would be nightmarish...
Steve Jessop
2009-09-09 00:48:13
+3
A:
Why not
sizeof "[]" - 1;
(minus one for the trailing null. You could do sizeof "[]" - sizeof '\0', but sizeof '\0' is often sizeof( int ) in C, and "- 1 " is perfectly readable.)
William Pursell
2009-09-08 02:48:29
could be fixed for wide strings. Something like: `(sizeof(L"[]") / sizeof(L"")) - 1`
Evan Teran
2009-09-08 02:55:10
@Evan Teran: yes, but you should use macros to make it more readable. IMHO macros it's more C style but not C++
Dmitriy
2009-09-08 02:59:11
I agree, was just pointing out that this technique was usable for wide strings. I was not endorsing it :-P.
Evan Teran
2009-09-08 03:01:35
A:
#define TWO 2
#define STRING_LENGTH 2
/* ... etc ... */
Seriously, why go through all this hassle just to avoid typing a 2? I honestly think you're making your code less readable, and other programmers are going to stare at you like you're snorting the used coffee from the filter.
Jed Smith
2009-09-08 02:49:46
it's just example. In real code it looks like "some string". Are you going to count number of characters in this case? :)
Dmitriy
2009-09-08 02:53:18
@Jed Smith: :) Are you sure that you don't forget to change the macro definition if the string changes?
Dmitriy
2009-09-08 03:16:28
@Jed Smith: How is "2" more readable/understandable then sizeof("[]") ?
Malkocoglu
2009-09-08 06:13:49
@Jed: Hey, if it's only a 2, then why bother? When it's "some string I might change and I don't want to bother eyeballing the length of" it's a different story. OTOH, strlen() is only a problem when it's really a problem. Assuming it's a problem is a form of guessing.
Mike Dunlavey
2009-09-08 13:54:43
+1
A:
I think most compilers will optimize it away when optimizations are enabled. If they're disabled, it might slow your program down much more than necessary.
I would prefer your template functions, as they're guaranteed to not call strlen at runtime.
Of course, rather than writing separate functions for char and wchar_t, you could add another template argument, and get a function which works for any type:
template <typename Char_t, int len>
int static_strlen(const Char_t (&)[N] array){
return len / sizeof(Char_t) - 1;
}
(As already mentioned in comments, this will give funny results if passed an array of ints, but are you likely to do that? It's meant for strings, after all)
A final note, the name _strlen is bad. All name at namespace scope beginning with an underscore are reserved to the implementation. You risk some nasty naming conflicts.
By the way, why is "[]" less of a magic constant than 2 is?
In both cases, it is a literal that has to be changed if the format of the string it is compared to changes.
jalf
2009-09-08 11:01:26