erase_remove.md

Erase-Remove Idiom

Elements from std::string and std::vector can be removed using the erase-remove idiom.

Example: Removing Every Zero From A Vector

std::vector<int> v{ 0, 2, 3, 0, 5, 7, 0 };
v.erase( std::remove(v.begin(), v.end(), 0),
        v.end() );
// v is now {2, 3, 5, 7}

Explanation

std::remove accepts iterators to the range, and the element to be removed. It will move all remaining elements to the beginning. In our example, the result is: {2, 3, 5, 7, ?, ?, ?}, where ? represents an unspecified state. The result is an iterator to the first "removed" element, i.e. the first ?.

std::vector::erase then reduces the size of the vector by erasing all "garbage" elements at the end.

Alternatives Since C++20

There now exists a std::erase function which does both steps in one function. However, the idiom can be applied to other algorithms like std::unique, so it is still useful.