std::forward_list<T,Allocator>::forward_list

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C++
 
Containers library
Sequence
(C++11)
(C++11)
Associative
Unordered associative
(C++11)
(C++11)
(C++11)
(C++11)
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(C++20)
 
std::forward_list
Member functions
forward_list::forward_list
Element access
Iterators
Capacity
Modifiers
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Non-member functions
(C++20)(C++20)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Deduction guides(C++17)
 
forward_list();
 (1)
explicit forward_list( const Allocator& alloc );
 (2)
forward_list( size_type count,

              const T& value,

              const Allocator& alloc = Allocator());
 (3) (since C++11)
(4)
explicit forward_list( size_type count );
 (since C++11)
(until C++14)
explicit forward_list( size_type count, const Allocator& alloc = Allocator() );
 (since C++14)
template< class InputIt >

forward_list( InputIt first, InputIt last,

              const Allocator& alloc = Allocator() );
 (5) (since C++11)
forward_list( const forward_list& other );
 (6) (since C++11)
forward_list( const forward_list& other, const Allocator& alloc );
 (7) (since C++11)
forward_list( forward_list&& other );
 (8) (since C++11)
forward_list( forward_list&& other, const Allocator& alloc );
 (9) (since C++11)
forward_list( std::initializer_list<T> init,
              const Allocator& alloc = Allocator() );
 (10) (since C++11)

Constructs a new container from a variety of data sources, optionally using a user supplied allocator alloc.

1) Default constructor. Constructs an empty container with a default-constructed allocator.
2) Constructs an empty container with the given allocator alloc.
3) Constructs the container with count copies of elements with value value.
4) Constructs the container with count default-inserted instances of T. No copies are made.
5) Constructs the container with the contents of the range [first, last).

This constructor has the same effect as forward_list(static_cast<size_type>(first), static_cast<value_type>(last), a) if InputIt is an integral type.

(until C++11)

This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator, to avoid ambiguity with the overload (3).

(since C++11)
6) Copy constructor. Constructs the container with the copy of the contents of other.

The allocator is obtained as if by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction(
    other.get_allocator()).

(since C++11)
7) Constructs the container with the copy of the contents of other, using alloc as the allocator.

The template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

(since C++23)
8) Move constructor. Constructs the container with the contents of other using move semantics. Allocator is obtained by move-construction from the allocator belonging to other.
9) Allocator-extended move constructor. Using alloc as the allocator for the new container, moving the contents from other; if alloc != other.get_allocator(), this results in an element-wise move.

The template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

(since C++23)
10) Constructs the container with the contents of the initializer list init.

Parameters

alloc - allocator to use for all memory allocations of this container
count - the size of the container
value - the value to initialize elements of the container with
first, last - the range to copy the elements from
other - another container to be used as source to initialize the elements of the container with
init - initializer list to initialize the elements of the container with

Complexity

1-2) Constant
3-4) Linear in count
5) Linear in distance between first and last
6-7) Linear in size of other
8) Constant.
9) Linear if alloc != other.get_allocator(), otherwise constant.
10) Linear in size of init.

Exceptions

Calls to Allocator::allocate may throw.

Notes

After container move construction (overload (8)), references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

Example

Run this code
#include <forward_list>
#include <string>
#include <iostream>
 
template<typename T>
std::ostream& operator<<(std::ostream& s, const std::forward_list<T>& v) 
{
    s.put('[');
    char comma[3] = {'\0', ' ', '\0'};
    for (const auto& e : v) {
        s << comma << e;
        comma[0] = ',';
    }
    return s << ']';
}
 
int main() 
{
    // c++11 initializer list syntax:
    std::forward_list<std::string> words1 {"the", "frogurt", "is", "also", "cursed"};
    std::cout << "words1: " << words1 << '\n';
 
    // words2 == words1
    std::forward_list<std::string> words2(words1.begin(), words1.end());
    std::cout << "words2: " << words2 << '\n';
 
    // words3 == words1
    std::forward_list<std::string> words3(words1);
    std::cout << "words3: " << words3 << '\n';
 
    // words4 is {"Mo", "Mo", "Mo", "Mo", "Mo"}
    std::forward_list<std::string> words4(5, "Mo");
    std::cout << "words4: " << words4 << '\n';
}

Output: 

words1: [the, frogurt, is, also, cursed]
words2: [the, frogurt, is, also, cursed]
words3: [the, frogurt, is, also, cursed]
words4: [Mo, Mo, Mo, Mo, Mo]

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2193 C++11 the default constructor is explicit made non-explicit

See also

(C++11)
 assigns values to the container
(public member function)
(C++11)
 assigns values to the container
(public member function)
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