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POFancy way to read a file in C++ : strange performance issue
 

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  1. POFancy way to read a file in C++ : strange performance issue
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    <p>The usual way to read a file in C++ is this one:</p> <pre><code>std::ifstream file("file.txt", std::ios::binary | std::ios::ate); std::vector&lt;char&gt; data(file.tellg()); file.seekg(0, std::ios::beg); file.read(data.data(), data.size()); </code></pre> <p>Reading a 1.6 MB file is almost instant.</p> <p>But recently, I discovered <a href="http://cplusplus.com/reference/std/iterator/istream_iterator/" rel="noreferrer">std::istream_iterator</a> and wanted to try it in order to code a beautiful one-line way to read the content of a file. Like this:</p> <pre><code>std::vector&lt;char&gt; data(std::istream_iterator&lt;char&gt;(std::ifstream("file.txt", std::ios::binary)), std::istream_iterator&lt;char&gt;()); </code></pre> <p>The code is nice, but <strong>very</strong> slow. It takes about 2/3 seconds to read the same 1.6 MB file. I understand that it may not be the best way to read a file, but why is it <strong>so</strong> slow?</p> <p>Reading a file in a classical way goes like this (I'm talking only about the read function):</p> <ul> <li>the istream contains a <a href="http://cplusplus.com/reference/iostream/filebuf/" rel="noreferrer">filebuf</a> which contains a block of data from the file</li> <li>the read function calls <a href="http://cplusplus.com/reference/iostream/streambuf/sgetn/" rel="noreferrer">sgetn</a> from the filebuf, which copies the chars one by one (no memcpy) from the inside buffer to "data"'s buffer</li> <li>when the data inside of the filebuf is entirely read, the filebuf reads the next block from the file</li> </ul> <p>When you read a file using istream_iterator, it goes like this:</p> <ul> <li>the vector calls *iterator to get the next char (this simply reads a variable), adds it to the end and increases its own size</li> <li>if the vector's allocated space is full (which happens not so often), a relocation is performed</li> <li>then it calls ++iterator which reads the next char from the stream (operator >> with a char parameter, which certainly just calls the filebuf's sbumpc function)</li> <li>finally it compares the iterator with the end iterator, which is done by comparing two pointers</li> </ul> <p>I must admit that the second way is not very efficient, but it's at least 200 times slower than the first way, how is that possible?</p> <p>I thought that the performance killer was the relocations or the insert, but I tried creating an entire vector and calling std::copy, and it's just as slow.</p> <pre><code>// also very slow: std::vector&lt;char&gt; data2(1730608); std::copy(std::istream_iterator&lt;char&gt;(std::ifstream("file.txt", std::ios::binary)), std::istream_iterator&lt;char&gt;(), data2.begin()); </code></pre>
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    1. CO"I thought that the performance killer was the relocations or the insert" - this is why you need to rely on profiling.
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      1. USMark Ransom
 

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