Why is C iostream Performance Sometimes Slow, and When Are Alternatives Necessary?

Slow Performance of C Standard Library iostreams

While the C standard library's iostreams may not explicitly mandate poor performance, the technical report on C Performance suggests that certain aspects of its processing can lead to inefficiencies.

Standard-Mandated Inefficiencies

According to the report, iostreams implementation typically involves distributing processing over multiple facets. This can result in inherent inefficiencies due to the overhead associated with each facet's operation.

Facets and Write()

While facets may not be directly used in the write() operation, profiling your ostringstream code reveals that significant time is spent in std::basic_streambuf::xsputn(). This function performs various checks and updates, leading to additional processing overhead.

Optimizations in Modern Compilers

The report mentions that some compilers may employ preprocessing or advanced linking techniques to mitigate the inefficiencies associated with iostreams. However, it's uncertain how many current compilers have incorporated these optimizations.

Alternative Implementations

In practice, compilers like Visual C 2010 and gcc 4.3.4 exhibit competitive performance with manual buffer management when using vector and simple iterators. This suggests that some implementations may provide optimized solutions for iostreams.

Practical Considerations

The observed slow performance of iostreams in your tests stems from the worst-case scenario of writing small chunks of data repeatedly. In real-world applications, dealing with larger data blocks reduces the impact of overheads and allows iostreams to fully utilize its memory-safe and type-safe design advantages.

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