Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.
PrevUpHomeNext

Performance

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 components (small-string optimization, 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-11.2.0 on 64-bit Ubuntu 22.04, tests produced the following results:

str-to-int: spirit/strtol/lcast/scanf/stream/charconv= 0.15/0.13/1.99/0.86/0.67/0.16 seconds.
str-to-lng: spirit/strtol/lcast/scanf/stream/charconv= 0.12/0.15/2.04/0.88/0.66/0.12 seconds.
str-to-dbl: spirit/strtol/lcast/scanf/stream/charconv= 0.15/0.53/5.12/1.06/1.68/0.19 seconds.
int-to-str: spirit/strtol/lcast/prntf/stream/charconv= 0.39/0.29/0.43/1.09/0.68/0.29 seconds.
lng-to-str: spirit/strtol/lcast/prntf/stream/charconv= 0.39/0.29/0.45/1.11/0.68/0.31 seconds.
dbl-to-str: spirit/strtol/lcast/prntf/stream/charconv= 1.12/1.17/7.34/5.05/4.65/1.07 seconds.

Based on the above, all things considered, I tend to conclude that:

  • the Spirit.Qi-based, strtol-inspired and charconv-based converters were the fastest for string to basic (int, double) conversions. So, they might be good candidates for the tasks doing that kind of conversions (with Spirit.Qi conversion-related limitations in mind);
  • 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 boost::lexical_cast-based converter was consistently and noticeably the slowest (due to its underlying design);
  • the charconv-based and the strtol-inspired converters were overall the fastest and with formatting support might be attractive all-rounders.

For user-defined types converters were tested with the following results:

str-to-user-type: lcast/stream/strtol/charconv=0.13/0.05/0.01/0.02 seconds.
user-type-to-str: lcast/stream/strtol/charconv=0.11/0.03/0.01/0.01 seconds.

To provide string-to-user-type and user-type-to-string conversions the first two deploy the same standard std::iostream library. Still, boost::cnv::cstream considerably outperforms boost::lexical_cast in these tests. That reflects 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 and boost::cnv::charconv support for user types has been implemented similarly but without the std::stream-related overhead. That resulted in considerably better 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.


PrevUpHomeNext