How to Shrink a `std::vector`\'s Capacity: The \'Swap Trick\' Explained

How to Reduce the Capacity of a std::Vector

This question, which has been asked previously, addresses a specific issue faced by many programmers when working with std::vectors. It seeks a method to downsize the capacity of a vector, reducing its excess memory allocation after its contents have been reduced.

Answer:

The recommended solution, as suggested in Effective STL by Scott Meyers (Item 17), is to employ the "swap trick" to trim the excess capacity. This method involves creating a new vector with the desired reduced capacity and swapping its contents with the original vector.

<code class="cpp">vector&lt;Person&gt;(persons).swap(persons);</code>

After performing this swap, the original vector, now empty, will be deallocated, and the new copy will take its place with its reduced capacity that is a perfect fit for its current contents.

Underlying Principle:

This technique leverages a key aspect of the vector's copy constructor behavior. When copying elements from a vector, the new vector allocates only enough memory to accommodate the copied elements, regardless of the original vector's capacity. By swapping the new, downsized vector with the original, the excess capacity is effectively trimmed.

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