Performance Difference Between C and C++ Style File IO

You're using endl to print a newline. That is the problem here, as it does more than just printing a newline — endl also flushes the buffer which is an expensive operation (if you do that in each iteration).

Use \n if you mean so:

file << i << '\n';

And also, must compile your code in release mode (i.e turn on the optimizations).

No, C++ input/output is not substantially slower than C’s – if anything, a modern implementation should be slightly faster on formatted input/output since it doesn’t need to parse a format string, and the formatting is instead determined at compile time through the chaining of the stream operators.

Here are a few caveats to consider in a benchmark:

• Compile with full optimisations (-O3) to get a fair comparison.
• A proper benchmark needs to estimate biases – in practice this means that you need to repeat your tests and interleave them. At the moment your code isn’t robust to disturbances from background processes. You should also report a summary statistic of the repeated runs to catch outliers that distort the estimates.
• Disable C++ stream synchronisation with C streams (std::ios_base::sync_with_stdio(false);)
• Use '\n' instead of the (flushing) std::endl
• Don’t use register declarations – it simply makes no difference and modern compilers probably ignore it anyway.

When working with large files with fstream, make sure to set a stream buffer >0.

Counterintuitively, disabling stream buffering dramatically reduces performance. At least the MSVC 2015 implementation copies 1 char at a time to the filebuf when no buffer was set (see streambuf::xsputn), which can make your application CPU-bound, which will result in lower I/O rates.

const size_t bufsize = 256*1024;char buf[bufsize];mystream.rdbuf()->pubsetbuf(buf, bufsize);

You can find a complete sample application here.