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    <p>Don't use C++ stl strings and <code>getline</code> ( or C's fgets), just C style raw pointers and either block read in page-size chunks or mmap the file. </p> <p>Then scan the block at the native word size of your system ( ie either <code>uint32_t</code> or <code>uint64_t</code>) using one of the <a href="http://aggregate.org/MAGIC/#SIMD%20Within%20A%20Register%20(SWAR)%20Operations" rel="noreferrer">magic algorithms</a> 'SIMD Within A Register (SWAR) Operations' for testing the bytes within the word. An example is <a href="http://www.tincancamera.com/examples/wide-finder/src/tbray9.cpp.txt" rel="noreferrer">here</a>; the loop with the <code>0x0a0a0a0a0a0a0a0aLL</code> in it scans for line breaks. ( that code gets to around 5 cycles per input byte matching a regex on each line of a file )</p> <p>If the file is only a few tens or a hundred or so megabytes, and it keeps growing (ie something keeps writing to it), then there's a good likelihood that linux has it cached in memory, so it won't be disk IO limited, but memory bandwidth limited. </p> <p>If the file is only ever being appended to, you could also remember the number of lines and previous length, and start from there.</p> <hr> <p>It has been pointed out that you could use mmap with C++ stl algorithms, and create a functor to pass to std::foreach. I suggested that you shouldn't do it not because you can't do it that way, but there is no gain in writing the extra code to do so. Or you can use boost's mmapped iterator, which handles it all for you; but for the problem the code I linked to was written for this was much, much slower, and the question was about speed not style. </p>
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    1. POFastest way to find the number of lines in a text (C++)
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    1. COThere is no reason you can't use mmap or reading blocks in C++.
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    2. COYou have to jump through a lot of hoops to convert between stl containers and strings and raw mmapped memory, which often involves copying and indirection, rather than just calling the functions and using the memory directly using the C subset of C++.
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    3. COThere is of course no "C subset" of C++. Just because you don't use a std container or string does not make it "not C++" in some way.
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