erase_if() Function in C++

The std::erase_if() is a utility introduced in C++20 that is used to remove elements from containers based on a specified condition. This function erases all elements that satisfy a given predicate from standard containers like std::vector, std::deque, std::list, std::forward_list, std::string, and associative containers.

In this article, we will learn about the erase_if() function, its syntax, usage scenarios, and practical examples to demonstrate how it can be used effectively in C++ programming.

Syntax of erase_if() in C++

erase_if(container, predicate);

Parameters of std::erase_if()

• container is the container from which elements will be removed.
• predicate is a unary function that determines whether an element should be erased as it return a true if the element should be removed and false otherwise.

Return Value of std::erase_if()

The std::erase_if() function returns the number of elements removed from the container.

Supported Containers

The erase_if() function is supported for the following standard containers:

• Sequence Containers: std::vector, std::deque, std::list, std::forward_list
• Associative Containers: std::map, std::set, std::multimap, std::multiset
• Unordered Containers: std::unordered_map, std::unordered_set, std::unordered_multimap, std::unordered_multiset

Examples of std::erase_if() in C++

The below examples demonstrates how we can use the erase_if() function to remove elements from a container based on a condition.

Example 1: Removing Negative Numbers from a Vector

Output

Number of elements removed: 3
Remaining elements: 1 3 5 7 

Time Complexity: O(n), where n is the number of elements in the vector.
Auxiliary Space: O(1)

Example 2: Removing Entries from a Map Based on Value

Output

Number of elements removed: 2
Remaining elements: {2, 200} {3, 150} {5, 250} 

Time Complexity: The time complexity of the std::erase_if() function depends on the container type:

• For std::vector and std::deque, it is O(n).
• For std::list and std::forward_list, it is O(n) with bidirectional iterators.
• For associative containers like std::set, std::map, std::unordered_set, and std::unordered_map, it depends on the underlying implementation but generally is O(n) for linear search.

Auxiliary Space: The auxiliary space required by std::erase_if() is O(1) as it operates in-place within the container.