std::find specialization | Microsoft Connect

std::find specialization by PowerGamer1

Closed

as Fixed

Type:	Bug
ID:	757385
Opened:	8/7/2012 12:44:19 PM
Access Restriction:	Public

Workaround(s)

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User(s) can reproduce this bug

When using std::find with "char" data type, in many cases for-based implementation of std::find is invoked instead of memchr one. To force compiler to use memchr-based implementation of std::find in such a case (see example program) one must use a total of THREE static_casts! This is completely counter-intuitive and absurd.

In all situations when using "char" data type std::find should use memchr.

Details (expand)

Visual Studio 2012 RC

#include <algorithm>
int main()
{
char str[] = "test";
std::find(str, str+sizeof(str), 's'); // for-based std::find instead of memchr-based
std::find(static_cast<const char*>(str), static_cast<const char*>(str)+sizeof(str), static_cast<int>('s')); // forcing to use memchr-based implementation
}

English

Windows 7

English

Single step program in debug to see that first call of std::find uses generic for-based implementation of std::find.

Not only the second but first call to std::find should use memchr specialization of std::find.

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Hi,

Thanks for reporting this bug. We've fixed it, and the fix will be available in the next update to our C++ Standard Library implementation.

I rewrote find() so it's maximally aggressive about activating the memchr() optimization. Specifically, it's now activated when all of these stars align:

* The given iterators are pointers, or things that the STL knows how to unwrap to pointers (this applies to std::vector, std::string, and std::array iterators).

* The elements are char, signed char, or unsigned char. They can be either modifiable or const.

* The given value is integral (of any signedness and any width).

I also fixed a surprising correctness bug in the old implementation. Given a signed char range containing -1, find() should report that 255 is not found within the range, because static_cast<signed char>(-1) != 255. memchr() happily considers them to be equal. We now perform range checks to adapt memchr()'s behavior to find()'s. This includes the special case where, for example, 0xFFFFFFFFUL is outside the [-128, 127] range of signed char, but is equal to the signed char -1 anyways (thanks to the usual arithmetic conversions).

Note: Connect doesn't notify me about comments. If you have any further questions, please E-mail me.

Stephan T. Lavavej
Senior Developer - Visual C++ Libraries
stl@microsoft.com