erase by iterator vs. erase by key in C++
Overview
erasing by iterator in std::maps is typically faster than erasing by key.
Erasing by Iterator:
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When you erase an element using an iterator, the map already has a direct reference to the node that contains the element to be erased.
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The iterator is a pointer to the element (node) in the underlying tree structure (which is usually a red-black tree for std::map).
Erasing by Key:
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When you erase an element by key, std::map first has to search for the key.
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It does this by traversing the red-black tree to find the node that matches the key.
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After finding the node, it performs the same operation as erase(iterator), but since it had to perform a search to locate the key, this is slower than directly accessing the iterator.
Benchmarks
Compile and run these 2 programs to see the performance difference.
map_erase_slow.cpp map_erase_fast.cpp
Both programs just create a map containing 1 millions integers, and then erase these 1 millions integers. As can be seen from the following results, erasing by iterators is much faster than erasing by key.
$g++ map_erase_slow.cpp -o map_erase_slow
$g++ map_erase_fast.cpp -o map_erase_fast
$time ./map_erase_slow
real 0m0.640s
user 0m0.624s
sys 0m0.016s
$time ./map_erase_fast
real 0m0.406s
user 0m0.386s
sys 0m0.020s
$time ./map_erase_fast
real 0m0.382s
user 0m0.374s
sys 0m0.008s
$time ./map_erase_slow
real 0m0.629s
user 0m0.617s
sys 0m0.012s
$time ./map_erase_slow
real 0m0.632s
user 0m0.623s
sys 0m0.009s
$time ./map_erase_fast
real 0m0.383s
user 0m0.366s
sys 0m0.017s