Why does erase-remove_if leave behind duplicate pairs when removing elements from a `std::vector`?

Erase-Remove_if Idiom for Pair Removal

When attempting to employ the erase-remove_if idiom to eliminate pairs from a std::vector>, a peculiar issue arises. Despite targeting pairs with a .first value of 4 for removal, the initial implementation leaves behind a duplicate pair:

stopPoints.erase(std::remove_if(stopPoints.begin(),
                                stopPoints.end(),
                                [&](const stopPointPair stopPoint)-> bool { return stopPoint.first == 4; }));

The root of the problem lies in the incomplete erasure process. std::erase_if only shifts matching elements to the vector's end; it does not remove them. To complete the removal, the correct approach is to use the iterator returned by std::remove_if as the starting point for the erasure:

stopPoints.erase(std::remove_if(stopPoints.begin(),
                                stopPoints.end(),
                                [](const stopPointPair stopPoint)-> bool 
                                       { return stopPoint.first == 4; }), 
                 stopPoints.end());

Understanding the Erase-Remove_if Mechanism:

• Element Swapping: std::remove_if swaps elements within the vector, pushing all mismatched elements toward the beginning. Matched elements end up at the vector's rear.
• Predicate Iteration: The predicate lambda expression determines which elements to remove. If the predicate returns true, the corresponding element is moved to the vector's end.
• Iterator Retrieval: std::remove_if returns an iterator pointing to the first element matching the predicate; this iterator marks the start of the elements to be removed.
• Vector Erasure: std::vector::erase invokes the range erasure operation, starting from the returned iterator and extending to the vector's end. This step removes all matched elements from the vector.

For further insights, refer to the Wikipedia article on the [Erase-Remove Idiom](https://en.wikipedia.org/wiki/Erase-remove_idiom).

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