Understanding the Difference: erase vs. remove

In the realm of C programming, std::erase and std::remove are two distinct functions that serve different purposes when it comes to modifying containers. While both functions can be used to eliminate elements from a container, they differ in their behavior.

Std::remove: Rearranging Elements vs. Deletion

Std::remove is an algorithm that operates on a range of elements and rearranges them within the container. It does not directly delete any elements but moves non-matching elements over matching ones. This process creates a cluster of matching elements at the beginning of the sequence and non-matching elements at the end.

Std::erase: Deleting Elements

On the other hand, std::erase is a function that removes specified elements from a container, effectively reducing its size. It takes a range of iterators as arguments and deletes all elements within that range, including the elements marked for removal.

Understanding the Output

In the code example provided, the following observations can be made:

1. Std::remove: When std::remove is used without std::erase, it simply rearranges the elements, leaving the vector's size unchanged. Therefore, iterating through the vector will result in the output of 2,2.
2. Std::erase: When std::erase is used in conjunction with std::remove, it removes the matching elements (in this case, the single occurrence of 1) and updates the vector's size accordingly. As a result, the output shows only 2.

Additional Notes on Std::remove

1. Usage Outside of Erase-Remove Idiom: While std::remove is commonly used with erase as part of the "erase-remove idiom," it can also be employed independently. It is useful in scenarios where the removal order is not crucial and the primary goal is to separate matching and non-matching elements within the container.
2. Rationale for Non-Deletion: The design of std::remove not involving deletion stems from its ability to work with arbitrary forward iterators. Such iterators may not have the ability to delete elements, hence the limited functionality of std::remove.