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Converters Compared
Boost.Convert Overhead
The Bigger Picture

The performance of Boost.Convert depends entirely on the performance of the converter deployed. A few converters have been tested for string conversions to basic types and to a user-defined type.

In turn, the performance of every particular converter depends on the platform, the compiler used and the particular implementation of the underlying conversion component (std::stream, printf, Boost.Spirit, etc.). Consequently, the results below are only an approximate indication of relative performance of the mentioned converters on the tested platforms.

When compiled with gcc-5.4.0 on 64-bit Ubuntu 16.04, tests produced the following results:

str-to-int: spirit/strtol/lcast/scanf/stream= 0.27/ 0.35/  0.92/  2.11/  2.09 seconds.
str-to-lng: spirit/strtol/lcast/scanf/stream= 0.69/ 0.31/  1.28/  2.07/  2.50 seconds.
str-to-dbl: spirit/strtol/lcast/scanf/stream= 0.73/ 1.06/  7.95/  2.87/  5.10 seconds.
int-to-str: spirit/strtol/lcast/prntf/stream= 1.96/ 1.39/  2.52/  3.49/  2.58 seconds.
lng-to-str: spirit/strtol/lcast/prntf/stream= 2.45/ 1.51/  2.32/  3.30/  2.63 seconds.
dbl-to-str: spirit/strtol/lcast/prntf/stream= 6.62/ 4.46/ 28.69/ 20.60/ 14.16 seconds.

Based on the results, all things considered, I tend to conclude that there is no clear winner:

  • the Spirit.Qi-based converter was the fastest for string to basic (int, double) conversions. So, it might be a good candidate for the tasks predominantly doing that kind of conversions (with Spirit.Qi conversion-related limitations in mind); Spirit.Karma's to-string performance did not seem as impressive;
  • the std::iostream-based converter was comparatively slow. Still, given its maturity, availability and formatting support, it might be an option to consider if conversion performance is not your primary concern;
  • the strtol-inspired converter was reasonably fast and with formatting support might be an attractive all-rounder. It should be noted that it is nowhere as mature as boost::cnv::lexical_cast or boost::cnv::stream. So, bugs are to be expected.

For user-defined types cnv::lexical_cast, cnv::cstream and cnv::strtol were tested with the following results:

str-to-user-type: lcast/stream/strtol=0.36/0.18/0.07 seconds.
user-type-to-str: lcast/stream/strtol=0.58/0.09/0.06 seconds.

To provide string-to-user-type and user-type-to-string conversions the first two deploy the same standard std::iostream library. However, boost::cnv::cstream considerably outperforms boost::lexical_cast in these tests. The results reflect different underlying designs. Namely, the standard Boost.Convert deployment pattern is to create a converter or converters once and then re-use them. boost::lexical_cast, on the other hand, creates and then destroys a std::stream instance every time the function is called and the boost::lexical_cast performance table indicates that the "std::stringstream with construction" operation is considerably more expensive compared to "std::stringstream without construction".

boost::cnv::strtol support for user types has been implemented similarly but without the std::stream-related overhead. That resulted in the best out-of-three performance results.

Based on the performance data, I tend to conclude that, given type-safety and benefits provided by the Boost.Convert framework, it (with appropriate converters) should probably be the first choice for conversion-related tasks.