Differences between std::views::filter() and std::remove_if()

std::views::filter() and std::remove_if() serve different purposes and have different performance characteristics.

std::views::filter() has the following characteristics:

• Purpose: Creates a view that includes only the elements that satisfy a given predicate.
• Non-Owning: Does not modify the original container. It simply provides a filtered view.
• Lazy Evaluation: Elements are evaluated on-demand, which can be more efficient if you only need to process part of the range.

The following program uses std::views::filter() to create a view of an underlying std::vector. The view contains only the odd numbers within the vector, but the vector itself is not modified:

1#include <iostream>
2#include <ranges>
3#include <vector>
4
5int main() {
6  std::vector<int> Numbers{1, 2, 3, 4, 5};
7
8  auto FilteredView = Numbers
9    | std::views::filter([](int x) {
10    return x % 2 == 1;
11  });
12
13  for (int Num : FilteredView) {
14    std::cout << Num << ", ";
15  }
16}

11, 3, 5,

std::remove_if() has these characteristics:

• Purpose: Rearranges the elements in the container so that the elements that do not satisfy the predicate are moved to the beginning, and returns an iterator to the new end.
• Owning: Modifies the original container. You often need to erase the elements that do not satisfy the predicate afterward.
• Eager Evaluation: Processes all elements at once, which can be less efficient if only part of the range is needed.

The following program uses std::remove_if() to move the even numbers to the end of a std::vector.

It then uses the erase() method to remove those elements from the container.

As a result, only the odd numbers remain in our std::vector:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> Numbers{1, 2, 3, 4, 5};
7
8  auto it = std::remove_if(
9    Numbers.begin(), Numbers.end(),
10    [](int x) { return x % 2 == 0; }
11  );
12
13  Numbers.erase(it, Numbers.end());
14
15  for (int Num : Numbers) {
16    std::cout << Num << ", ";
17  }
18}

11, 3, 5,

Performance Comparison

• Memory: std::views::filter() uses less memory since it doesn't create a new container but instead provides a view. std::remove_if() may require extra memory if elements are erased and the container is resized.
• Speed: std::views::filter() can be faster for partial processing due to lazy evaluation. std::remove_if() processes all elements eagerly, which may be less efficient for large datasets.
• Use Case: Use std::views::filter() when you need a non-modifying, lazy-evaluated view. Use std::remove_if() when you need to modify the container directly and remove elements.

Understanding these differences helps choose the right tool for specific use cases, balancing performance and functionality needs.