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    <p>I'm not sure that there's a <strong>best</strong> way, but using the two iterator constructor would be more idiomatic:</p> <pre><code>FileContainer container( (std::istreambuf_iterator&lt;char&gt;( testFile )), (std::istreambuf_iterator&lt;char&gt;()) ); </code></pre> <p>(I notice that you have the extra parentheses in your <code>assign</code>. They aren't necessary there, but they are when you use the constructor.)</p> <p>With regards to performance, it would be more efficient to pre-allocate the data, something like:</p> <pre><code>FileContainer container( actualSizeOfFile ); std::copy( std::istreambuf_iterator&lt;char&gt;( testFile ), std::istreambuf_iterator&lt;char&gt;(), container.begin() ); </code></pre> <p>This is slightly dangerous; if your estimation is too small, you'll encounter undefined behavior. To avoid this, you could also do:</p> <pre><code>FileContainer container; container.reserve( estimatedSizeOfFile ); container.insert( container.begin(), std::istreambuf_iterator&lt;char&gt;( testFile ), std::istreambuf_iterator&lt;char&gt;() ); </code></pre> <p>Which of these two is faster will depend on the implementation; the last time I measured (with g++), the first was slightly faster, but if you're actually reading from file, the difference probably isn't measurable.</p> <p>The problem with these two methods is that, despite other answers, there is no portable way of finding the file size other than by actually reading the file. Non-portable methods exist for some systems (<code>fstat</code> under Unix), but on other systems, like Windows, there is <strong>no</strong> means of finding the exact number of <code>char</code> you can read from a text file. And of course, there's no guarantee that the results of <code>tellg()</code> will even convert to an integral type, and that if it does, that they won't be a magic cookie, with no numerical signification.</p> <p>Having said that, in practice, the use of <code>tellg()</code> suggested by other posters will often be "portable enough" (Windows and most Unix, at least), and the results will often be "close enough"; they'll usually be a little too high under Windows (since the results will count the carriage return characters which won't be read), but in a lot of cases, that's not a big problem. In the end, it's up to you to decide what your requirements are with regards to portability and precision of the size.</p>
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    1. POStreaming into vector?
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    1. USJames Kanze
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    1. POStreaming into vector?
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    1. COSeveral great answers, but i appreciate the detail in this answer and it makes me understand overall more clearly some of the possible solutions.
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      1. USDaniel Gratz
    2. COSince `std::istreambuf_iterator` appears to be an unformulated operation, it will return the correct size in Windows, not overestimate. It will insert the carriage returns as well. Also, `streampos` is convertible to `streamoff`, which converts to `streamsize`, which must be "one of the signed basic integral types". so `streamoff(tellg())` is guaranteed to convert to an integral type.
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      1. USMooing Duck
    3. CO@MooingDuck `std::istreambuf_iterator` does not return a size, and under Windows, it **will** read less `char`s than you get from determining the size of the file (since it won't read the carriage returns). And while not the case under Windows (or any of the Unices I know), `streamoff` can be a user defined type, in which case, the conversion `streampos` to `streamsize` would involve two user defined conversions (and thus not be implicit). And of course, as I said, the results of the conversion aren't necessarily numerically useful.
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      1. USJames Kanze
 

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