Remove vs Erase

The difference between std::remove() and std::erase() is crucial in understanding how to manipulate containers effectively in C++.

std::remove() is an algorithm found in the <algorithm> header. It does not change the size of the container but reorders the elements such that the elements to be "removed" are moved to the end.

It returns an iterator to the new end of the range, beyond which the elements are considered to be removed. However, the container itself still holds those elements.

On the other hand, erase() is a member function of containers like std::vector, std::list, and std::deque. It actually removes elements from the container and adjusts the container's size accordingly.

Here's an example to illustrate the difference:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> Source{1, 2, 3, 4, 5, 3, 6};
7
8  // Using std::remove()
9  auto NewEnd = std::remove(Source.begin(),
10    Source.end(), 3);
11
12  std::cout << "After std::remove(): ";
13  for (auto it = Source.begin();
14    it != Source.end(); ++it) {
15    std::cout << *it << ", ";
16  }
17  std::cout << "\nSize after std::remove(): "
18    << Source.size() << "\n";
19
20  // Using erase() to actually remove the elements
21  Source.erase(NewEnd, Source.end());
22
23  std::cout << "After erase(): ";
24  for (auto Num : Source) {
25    std::cout << Num << ", ";
26  }
27  std::cout << "\nSize after erase(): "
28    << Source.size() << "\n";
29}

1After std::remove(): 1, 2, 4, 5, 6, 3, 6,
2Size after std::remove(): 7
3After erase(): 1, 2, 4, 5, 6,
4Size after erase(): 5

In the example above:

• std::remove() moves the 3s to the end and returns an iterator to the new end, but the size of Source remains unchanged.
• erase() then actually removes the surplus elements and updates the container size.

Understanding the distinction between these two functions is critical for effective container management in C++.