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POHow to avoid reallocation using the STL (C++)
 

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  1. POHow to avoid reallocation using the STL (C++)
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    <p>This question is derived from the topic:</p> <p><a href="https://stackoverflow.com/questions/2280655/vector-reserve-c">vector reserve c++</a></p> <p>I am using a datastructure of the type <code>vector&lt;vector&lt;vector&lt;double&gt; &gt; &gt;</code>. It is not possible to know the size of each of these vector (except the outer one) before items (<code>double</code>s) are added. I can get an approximate size (upper bound) on the number of items in each "dimension".</p> <p>A solution with the shared pointers might be the way to go, but I would like to try a solution where the <code>vector&lt;vector&lt;vector&lt;double&gt; &gt; &gt;</code> simply has <code>.reserve()</code>ed enough space (or in some other way has allocated enough memory).</p> <p>Will <code>A.reserve(500)</code> (assumming 500 is the size or, alternatively an upper bound on the size) be enough to hold "2D" vectors of large size, say [1000][10000]?</p> <p>The reason for my question is mainly because I cannot see any way of reasonably estimating the size of the interior of <code>A</code> at the time of <code>.reserve(500)</code>.</p> <p>An example of my question:</p> <pre><code>vector&lt;vector&lt;vector&lt;int&gt; &gt; &gt; A; A.reserve(500+1); vector&lt;vector&lt;int&gt; &gt; temp2; vector&lt;int&gt; temp1 (666,666); for(int i=0;i&lt;500;i++) { A.push_back(temp2); for(int j=0; j&lt; 10000;j++) { A.back().push_back(temp1); } } </code></pre> <p>Will this ensure that no reallocation is done for A?</p> <p>If <code>temp2.reserve(100000)</code> and <code>temp1.reserve(1000)</code> were added at creation will this ensure no reallocation at all will occur at all?</p> <p>In the above please disregard the fact that memory could be wasted due to conservative <code>.reserve()</code> calls.</p> <p>Thank you all in advance!</p>
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    1. USTue Christensen
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    1. COIt looks like your example code didn't paste very well - I can't tell exactly what you're trying to do/avoid.
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    2. CONo indeed it didnt. It can however be seen in the link: http://stackoverflow.com/questions/2280655/vector-reserve-c
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