How to Reduce Excess Capacity in a std::vector: The Swap Trick Explained

How to Effectively Reduce Capacity in a std::vector

If you find yourself with a std::vector that has excess capacity, you may wonder if there's a way to downsize it to free up memory. While there's no direct method for this, there's a clever technique known as the "swap trick."

The Swap Trick

According to Item 17 from Scott Meyers' Effective STL, you can use the swap trick to trim excess capacity from a vector. Here's how it works:

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

After performing this swap, the vector named "persons" will be "shrunk to fit," meaning its capacity will be reduced to match the number of elements it contains.

How the Trick Works

This trick takes advantage of the fact that the vector's copy constructor allocates only the necessary amount of memory for the elements being copied. By creating a new vector with the same number of elements as the original vector, the copy constructor creates a new vector with a smaller capacity.

The swap method then swaps the contents and capacities of the original and new vectors. As a result, the original vector is left with a reduced capacity.

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