C++ Remove Algorithms: remove, unique & the Erase–Remove Idiom

Introduction

remove algorithms do not shrink the container — they compact elements and return an iterator to the new logical end. erase from there to end() actually removes excess elements.

---

1. remove

Typical pattern:

v.erase(std::remove(v.begin(), v.end(), value), v.end());

---

2. remove_if

Same pattern with a predicate (e.g. remove even numbers).

---

3. unique

Requires sorted order (or at least equal duplicates adjacent) to remove all duplicates globally — usually sort then unique then erase.

---

4. Examples

String whitespace cleanup, product filtering, duplicate removal — code matches Korean article; use erase-remove on std::string too.

---

5. Common pitfalls

• forgetting erase — stale size and junk tail values.
• unique without sort — only adjacent duplicates removed.
• multiple passes — combine conditions in one remove_if when possible.
• list — list::remove / remove_if member may be more appropriate (splice, no shifting).

---

6. C++20 std::erase / std::erase_if

Container-level erase helpers for brevity:

std::erase(v, 2);
std::erase_if(v, [](int x) { return x % 2 == 0; });

---

7. DataCleaner utility

The DataCleaner example (empty strings, duplicates, out-of-range, whitespace, normalize spaces) is unchanged in logic from the source article.

---

Summary

API	Behavior	Typical use
remove	value != kept	Erase-remove idiom
remove_if	pred false kept	Conditional erase
unique	adjacent dupes	After sort
erase	actually deletes	After remove

Next steps

• C++ Algorithm Replace
• C++ Algorithm Copy
• C++ Algorithm Sort

---

Related posts

• C++ Algorithm Copy
• C++ Algorithm Guide

Keywords

std::remove, remove_if, unique, erase-remove, std::erase, STL