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PORemove elements (and change size) of std::vector without affecting allocated memory
 

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  1. PORemove elements (and change size) of std::vector without affecting allocated memory
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    <p>I'm using code such as the following:</p> <pre><code>const int MY_SIZE = 100000; std::vector&lt;double&gt; v; v.reserve(MY_SIZE); // add no more than MY_SIZE elements to the vector f(v); v.clear(); // again, add no more than MY_SIZE elements to the vector f(v); v.clear(); // // etc... // </code></pre> <p>The point of my code is to store <code>MY_SIZE</code> <code>double</code>'s and then perform an operation <code>f(std::vector&lt;double&gt;)</code> on those elements. After I fill up the vector and perform the operation, I want to get rid of all the elements (and reset <code>std::vector::size()</code> to 0), and then add more elements. But, the key here is that I do <strong>not</strong> want to cause the space in memory allocated for the vector to be changed.</p> <p>Note than I'm never going to add more than <code>MY_SIZE</code> elements to <code>v</code>, so <code>v</code> should never need to reallocate more memory than was allocated by <code>v.reserve(MY_SIZE)</code>.</p> <p>So, when I call <code>v.clear()</code> in the above code, will it affect in any way the amount of space allocated by <code>v.reserve(MY_SIZE)</code> or the location in memory of <code>v.begin()</code>?</p> <hr> <p><strong>Related question</strong>: If I call <code>v.erase(v.begin(),v.begin()+v.size())</code>, will it affect in any way the amount of space allocated by <code>v.reserve(MY_SIZE)</code> or the location in memory of <code>v.begin()</code>?</p> <p>If I really just wanted to erase all the elements, I would call <code>clear()</code>. But I'm wondering about this related question because there are occasions when I need to erase only the first X elements of <code>v</code>, and on these occasions I want to keep the memory allocated by <code>v.reserve(MY_SIZE)</code> and I don't want the location of <code>v</code> to change.</p>
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    1. CONo. Clear does not reallocate. You can check `capacity()` to see it hasn't changed.
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    2. COThanks. That's what I was thinking, but wasn't certain from looking up `std::vector::clear()`. I'm sure the answer to this question (indirectly, if not directly) exists in the language documentation, but from what I looked at, I couldn't be certain.
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    3. CO@JonathanWakely: Are you sure about that? AFAIK most implementations do not change the capacity. However, it is allowed by the standard to shrink it.
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