std::erase,...orward_list)(3)

💅 NAME 💅

Bestie, this is the std::erase,std::erase_if(std::forward_list) page from the C++ Standard Library manual. It’s literally about erasing elements from a std::forward_list by value or by predicate. Iconic behavior, no cap. ✨

🦋 Synopsis 🦋

Defined in header <forward_list>.

template< class T, class Alloc, class U >
std::forward_list<T, Alloc>::size_type
erase( std::forward_list<T, Alloc>& c, const U& value );

(1) since C++20, until C++26.

template< class T, class Alloc, class U = T >
std::forward_list<T, Alloc>::size_type
erase( std::forward_list<T, Alloc>& c, const U& value );

(1) since C++26.

template< class T, class Alloc, class Pred >
std::forward_list<T, Alloc>::size_type
erase_if( std::forward_list<T, Alloc>& c, Pred pred );

(2) since C++20.

1. Erases all elements that compare equal to value from the container. Equivalent to return c.remove_if([&](auto& elem) { return elem == value; });.
2. Erases all elements that satisfy the predicate pred from the container. Equivalent to return c.remove_if(pred);.

🌸 Parameters 🌸

• c — container from which to erase.
• value — value to be removed.
• pred — unary predicate which returns true if the element should be erased.

The expression pred(v) must be convertible to bool for every argument v of type (possibly const) T, regardless of value category, and must not modify v. Thus, a parameter type of T& is not allowed, nor is T unless for T a move is equivalent to a copy (since C++11).

🎀 Return value 🎀

The number of erased elements.

✨ Complexity ✨

Linear.

🦋 Notes 🦋

Unlike std::forward_list::remove, erase accepts heterogeneous types and does not force a conversion to the container’s value type before invoking the == operator.

Feature-test macro vibes:

Feature-test macro                      Value   Std      Feature
__cpp_lib_algorithm_default_value_type  202403  (C++26)  List-initialization for algorithm (1)

💖 Example 💖

// Run this code

#include <complex>
#include <iostream>
#include <numeric>
#include <string_view>
#include <forward_list>

void println(std::string_view comment, const auto& c)
{
    std::cout << comment << "{ ";
    for (const auto& x : c)
        std::cout << x << ' ';
    std::cout << "}\n";
}

int main()
{
    std::forward_list<char> cnt(10);
    std::iota(cnt.begin(), cnt.end(), '0');
    println("Initially, cnt = ", cnt);

    std::erase(cnt, '3');
    println("After erase '3', cnt = ", cnt);

    auto erased = std::erase_if(cnt, [](char x) { return (x - '0') % 2 == 0; });
    println("After erase all even numbers, cnt = ", cnt);
    std::cout << "Erased even numbers: " << erased << '\n';

    std::forward_list<std::complex<double>> nums{{2, 2}, {4, 2}, {4, 8}, {4, 2}};
#ifdef __cpp_lib_algorithm_default_value_type
        std::erase(nums, {4, 2});
#else
        std::erase(nums, std::complex<double>{4, 2});
#endif
    println("After erase {4, 2}, nums = ", nums);
}

Output 🎀

Initially, cnt = { 0 1 2 3 4 5 6 7 8 9 }
After erase '3', cnt = { 0 1 2 4 5 6 7 8 9 }
After erase all even numbers, cnt = { 1 5 7 9 }
Erased even numbers: 5
After erase {4, 2}, nums = { (2,2) (4,8) }

👑 See also 👑

• remove — removes elements satisfying specific criteria
• remove_if (function template)
• remove — removes elements satisfying specific criteria
• remove_if (public member function)

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💖 That’s the whole page, bestie — source/version/date: http://cppreference.com, 2024.06.10, manual page std::erase,...orward_list)(3). 🫶