Why Does `std::sort` Avoid Calling a Custom `swap` Function for Small Ranges?

std::sort Can Avoid std::swap for Efficiency

Question:

Consider the following code using user-defined type A with custom swap function:

<code class="cpp">struct A {
    double a;
    double* b;
    bool operatora <p>When n is set to 20, the custom swap function is used and the array is sorted. However, when n is set to 4, the custom swap function is <strong>not</strong> called.</p>
<p><strong>Answer:</strong></p>
<p>For small ranges (such as when n is 4), std::sort implementations in GCC's stdlibc   (and other standard library implementations) <strong>switch to insertion sort</strong> for performance reasons.</p>
<p><strong>Insertion Sort Optimization:</strong></p>
<p>Insertion sort in GCC's implementation uses a different approach to swapping:</p>
<ol>
<li>It moves whole ranges of values at a time, using std::move_backward internally.</li>
<li>If the compiler's experimental C  11 features are not enabled, std::move_backward may use copying instead of moving.</li>
</ol>
<p>This optimization improves performance by avoiding unnecessary swaps. Instead of swapping elements individually, a portion of the array is shifted, effectively performing multiple swaps in one operation.</p>
<p><strong>Conclusion:</strong></p>
<p>When sorting small arrays, std::sort may use insertion sort to avoid invoking the custom swap function. This optimization can improve performance but should be considered when copying objects is costly.</p></code>

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