C++ Basics: Erase the Last Matching Value through a Reverse Iterator

Translate a reverse search result into the forward iterator required by vector::erase.

Prepare a scratch directory

Tested on Linux x86_64 with GCC 14.2.0. Install a C++20-capable compiler and standard-library development headers, with g++ on PATH. You also need Bash, a text editor, and the mktemp, rm, and rmdir utilities. No third-party library is required.

Keep the same terminal open. Stop if setup fails, before saving the source:

start_directory="$PWD"
work_directory="$(mktemp -d)" || exit 1
cd -- "$work_directory" || exit 1

Run the example

Save this as reverse-iterator-erase-demo.cpp:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

int main()
{
    std::vector<int> values{1, 2, 3, 2, 4};
    const auto found = std::find(values.rbegin(), values.rend(), 2);
    if (found != values.rend()) {
        values.erase(std::prev(found.base()));
    }
    std::cout << "Remaining:";
    for (int value : values) std::cout << ' ' << value;
    std::cout << '\n';
}

Compile and run it. The program runs only after compilation succeeds:

g++ -std=c++20 -Wall -Wextra -Werror -pedantic reverse-iterator-erase-demo.cpp -o reverse-iterator-erase-demo &&
./reverse-iterator-erase-demo

Observed output:

Remaining: 1 2 3 4

Read the result

The reverse search locates the final 2 in forward order. base() points one position after the element represented by a reverse iterator, so prev(base()) selects the actual element to erase. The earlier 2 remains.

Apply it carefully

Check against rend() first. Calling prev() on the base of rend() would step before begin(). Passing found.base() directly to erase would remove the wrong value or pass end(). Vector erasure invalidates iterators and references at or after the erased position, including this reverse search result; do not reuse it after the mutation.

The C++ working draft describes the relevant rules. This example builds in C++20 mode; the current working draft also documents later additions.

Clean up

Remove only the source and executable created here, then return to the starting directory:

rm -- reverse-iterator-erase-demo.cpp reverse-iterator-erase-demo
cd -- "$start_directory" || exit 1
rmdir -- "$work_directory"
unset start_directory work_directory

If the directory is not empty, inspect its remaining contents before removing anything else.

Acknowledgements

This post was created with assistance from AI.