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Asked: 2026-05-26T01:03:19+00:00

I am wondering whether all C++ formatting libraries eventually fall back to a *sprintf

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I am wondering whether “all” C++ formatting libraries eventually fall back to a *sprintf function to format numbers.

I am asking this because:

  • Looking at the iostreams library that comes with Visual C++, I can see that numbers input into a stream will eventuall be formatted with sprintf_s.
  • Boost.Format just uses the available iostreams library as far as I can tell.
  • FastFormat eventually uses vsprintf to format a number.

So, are there iostreams implementations that do not use *sprintf and do the formatting themselves? Are there other formatting libraries that do not forward formatting of numbers to *sprintf family of functions?

I would appreciate answers in the form of:

  • No: implementation XY uses ABC to format numbers
  • Yes: all other (e.g. iostreams) implementations I know (X, Y, Z) also forward number formatting to stdio, because …

Please avoid overly speculative answers.

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1 Answer

  1. Editorial Team

    Boost Spirit doesn’t use *printf, as can be seen from the code (real.hpp and int.hpp) and the benchmarks for e.g. ints and doubles.

    The benchmark pits Boost Spirit Karma’s generators against Boost.Format against sprintf and std::stringstream. Only for gcc compilers does the performance of sprintf come close in that benchmark. Otherwise, Boost Spirit is the clear winner.

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