


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 operator<(const A& rhs) const { return this->a < rhs.a; } }; void swap(A& lhs, A& rhs) { std::cerr << "My swap.\n"; // Custom swap function }</code>
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 not called.
Answer:
For small ranges (such as when n is 4), std::sort implementations in GCC's stdlibc (and other standard library implementations) switch to insertion sort for performance reasons.
Insertion Sort Optimization:
Insertion sort in GCC's implementation uses a different approach to swapping:
- It moves whole ranges of values at a time, using std::move_backward internally.
- If the compiler's experimental C 11 features are not enabled, std::move_backward may use copying instead of moving.
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.
Conclusion:
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.
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