When should you use an STL other than the one that comes with your compiler?

Question

I was curious about STL implementations outside of what's packaged with gcc or Visual Studio, so a quick Google search turned up a few results, such as:

• Apache stdcxx
• uSTL
• rdeSTL

Under what circumstances should one use an alternative standard template library?

For instance, Apache's page has a list including items such as "full conformance to the C++ standard" and "optimized for fast compiles and extremely small executable file sizes". If it's so good, why wouldn't it replace libstdc++?

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For the sake of completeness, here are some of the other STL implementations:

• STLPort
• STXXL (which is sort of special purpose, meant for large data sets that won't fit in memory)
• Dinkumware (commercial)
• SGI STL
• libstdc++ (GCC's implementation)

Answer 1

I never had to use an STL version other than the one packed with the compiler. But here are some points that come into my mind.

• Thread-safety: The STL from apache provides a compile switch to turn on/off some thread-safety features.
• Localization: Again the STL from apache comes with nice support for many different locales.
• Data structures: You might need a basic_string implementation that is based on COW (copy-on-write) and the STL version that came with your compiler doesn't offer that.
• Non-standard extensions: Particular features you like from some other STL implementations. For example, hash_map (and related) versions from Dinkumware (which ships with Visual Studio) have a significantly different design from hash_map (and related) from STLPort.
• Binary issues: Constraints in some environment (embedded software) due to code size. In such case, if you don't need the whole STL it could be interesting to use a reduced version.
• Performance: What if you discovered, after profiling, that the "other" STL implementation gives you significant better performance for a particular application. (With so many details concerning algorithms and data structures this could actually be possible.)
• Debug mode: Some STL implementation provide nice features for debugging. For instance, checking ranges of iterators.