Does the C++ standard mandate poor performance for iostreams, or am I just dealing with a poor implementation?

Question

Not answering the specifics of your question so much as the title: the 2006 Technical Report on C++ Performance has an interesting section on IOStreams (p.68). Most relevant to your question is in Section 6.1.2 ("Execution Speed"):

Since certain aspects of IOStreams processing are distributed over multiple facets, it appears that the Standard mandates an inefficient implementation. But this is not the case — by using some form of preprocessing, much of the work can be avoided. With a slightly smarter linker than is typically used, it is possible to remove some of these inefficiencies. This is discussed in §6.2.3 and §6.2.5.

Since the report was written in 2006 one would hope that many of the recommendations would have been incorporated into current compilers, but perhaps this is not the case.

As you mention, facets may not feature in write() (but I wouldn't assume that blindly). So what does feature? Running GProf on your ostringstream code compiled with GCC gives the following breakdown:

• 44.23% in std::basic_streambuf<char>::xsputn(char const*, int)
• 34.62% in std::ostream::write(char const*, int)
• 12.50% in main
• 6.73% in std::ostream::sentry::sentry(std::ostream&)
• 0.96% in std::string::_M_replace_safe(unsigned int, unsigned int, char const*, unsigned int)
• 0.96% in std::basic_ostringstream<char>::basic_ostringstream(std::_Ios_Openmode)
• 0.00% in std::fpos<int>::fpos(long long)

So the bulk of the time is spent in xsputn, which eventually calls std::copy() after lots of checking and updating of cursor positions and buffers (have a look in c++\bits\streambuf.tcc for the details).

My take on this is that you've focused on the worst-case situation. All the checking that is performed would be a small fraction of the total work done if you were dealing with reasonably large chunks of data. But your code is shifting data in four bytes at a time, and incurring all the extra costs each time. Clearly one would avoid doing so in a real-life situation - consider how negligible the penalty would have been if write was called on an array of 1m ints instead of on 1m times on one int. And in a real-life situation one would really appreciate the important features of IOStreams, namely its memory-safe and type-safe design. Such benefits come at a price, and you've written a test which makes these costs dominate the execution time.

I'm rather disappointed in the Visual Studio users out there, who rather had a gimme on this one:

• In the Visual Studio implementation of ostream, the sentry object (which is required by the standard) enters a critical section protecting the streambuf (which is not required). This doesn't seem to be optional, so you pay the cost of thread synchronization even for a local stream used by a single thread, which has no need for synchronization.

This hurts code that uses ostringstream to format messages pretty severely. Using the stringbuf directly avoids the use of sentry, but the formatted insertion operators can't work directly on streambufs. For Visual C++ 2010, the critical section is slowing down ostringstream::write by a factor of three vs the underlying stringbuf::sputn call.

Looking at beldaz's profiler data on newlib, it seems clear that gcc's sentry doesn't do anything crazy like this. ostringstream::write under gcc only takes about 50% longer than stringbuf::sputn, but stringbuf itself is much slower than under VC++. And both still compare very unfavorably to using a vector<char> for I/O buffering, although not by the same margin as under VC++.

The problem you see is all in the overhead around each call to write(). Each level of abstraction that you add (char[] -> vector -> string -> ostringstream) adds a few more function call/returns and other housekeeping guff that - if you call it a million times - adds up.

I modified two of the examples on ideone to write ten ints at a time. The ostringstream time went from 53 to 6 ms (almost 10 x improvement) while the char loop improved (3.7 to 1.5) - useful, but only by a factor of two.

If you're that concerned about performance then you need to choose the right tool for the job. ostringstream is useful and flexible, but there's a penalty for using it the way you're trying to. char[] is harder work, but the performance gains can be great (remember the gcc will probably inline the memcpys for you as well).

In short, ostringstream isn't broken, but the closer you get to the metal the faster your code will run. Assembler still has advantages for some folk.

To get better performance you have to understand how the containers you are using work. In your char[] array example, the array of the required size is allocated in advance. In your vector and ostringstream example you are forcing the objects to repeatedly allocate and reallocate and possibly copy data many times as the object grows.

With std::vector this is easly resolved by initialising the size of the vector to the final size as you did the char array; instead you rather unfairly cripple the performance by resizing to zero! That is hardly a fair comparison.

With respect to ostringstream, preallocating the space is not possible, I would suggest that it is an inappropruate use. The class has far greater utility than a simple char array, but if you don't need that utility, then don't use it, because you will pay the overhead in any case. Instead it should be used for what it is good for - formatting data into a string. C++ provides a wide range of containers and an ostringstram is amongst the least appropriate for this purpose.

In the case of the vector and ostringstream you get protection from buffer overrun, you don't get that with a char array, and that protection does not come for free.