On 11/23/2011 11:55 AM, Olaf van der Spek wrote:
On Wed, Nov 23, 2011 at 5:55 PM, Matthew Chambers wrote:
On Oct 13, 2011 Michel Morin wrote:
lexical_cast made optimizations for some types (such as `std::string`) to achieve better performance. But, from my experience, it seems that lexical_cast does not make any optimization for `boost::iterator_range<std::string::iterator>`.
Is there any plan to add optimization for such iterator ranges? Currently, - `lexical_cast<int>(std::string(iter_rng.begin(), iter_rng.end()))` is faster than `lexical_cast<int>(iter_rng)`.
Why's that?
Why is it faster? Presumably because lexical_cast has optimized specializations for std::string but not for iterator_range<std::string::iterator>. So creating a new string from the range invokes the optimized specialization and ends up being faster.
- Assuming it is safe to use `std::atoi`, `std::atoi(&iter_rng.front())` is faster than both of the above two methods.
Is front() guaranteed to return a 0-terminated string?
Certainly not. I consider that caveat to be part of the "assuming it is safe" precondition. He's just saying that there is potential to be faster. Although he didn't compare lexical_cast<int> on a string without copying the string, which should be comparable to atoi. I put together a test (http://codepad.org/lKxMXOwP): 1000000 iterations of "123": atoi: 0.0695092 seconds strtol: 0.0652839 seconds Spirit: 0.546789 seconds lexical_cast(string): 0.615594 seconds lexical_cast(iterator_range): 4.25534 seconds lexical_cast(iterator_range->string): 1.07405 seconds 1000000 iterations of "123567890": atoi: 0.0856611 seconds strtol: 0.0829571 seconds Spirit: 0.905017 seconds lexical_cast(string): 0.762137 seconds lexical_cast(iterator_range): 7.46185 seconds lexical_cast(iterator_range->string): 1.23397 seconds 1000000 iterations of "1.23456": atof: 0.520452 seconds strtod: 0.519113 seconds Spirit: 1.02579 seconds lexical_cast(string): 2.93515 seconds lexical_cast(iterator_range): 6.62648 seconds lexical_cast(iterator_range->string): 3.73831 seconds 1000000 iterations of "1.23456789e42": atof: 0.711819 seconds strtod: 0.719144 seconds Spirit: 1.48992 seconds lexical_cast(string): 3.31997 seconds lexical_cast(iterator_range): 9.87004 seconds lexical_cast(iterator_range->string): 4.20932 seconds -Matt