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

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C++
Containers library
std::forward_list
Member functions
Non-member functions
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20)
Deduction guides (C++17)
forward_list ( ) : forward_list ( Allocator ( ) ) { }
(1) (constexpr since C++26)
explicit forward_list ( const Allocator & alloc ) ;
(2) (constexpr since C++26)
explicit forward_list ( size_type count,
const Allocator & alloc = Allocator ( ) ) ;
(3) (constexpr since C++26)
forward_list ( size_type count, const T & value,
const Allocator & alloc = Allocator ( ) ) ;
(4) (constexpr since C++26)
template < class InputIt >

forward_list ( InputIt first, InputIt last,

const Allocator & alloc = Allocator ( ) ) ;
(5) (constexpr since C++26)
template < container-compatible-range < T > R >

forward_list ( std:: from_range_t , R && rg,

const Allocator & alloc = Allocator ( ) ) ;
(6) (since C++23)
(constexpr since C++26)
forward_list ( const forward_list & other ) ;
(7) (constexpr since C++26)
forward_list ( forward_list && other ) ;
(8) (constexpr since C++26)
(9)
forward_list ( const forward_list & other, const Allocator & alloc ) ;
(until C++23)
forward_list ( const forward_list & other,
const std:: type_identity_t < Allocator > & alloc ) ;
(since C++23)
(constexpr since C++26)
(10)
forward_list ( forward_list && other, const Allocator & alloc ) ;
(until C++23)
forward_list ( forward_list && other,
const std:: type_identity_t < Allocator > & alloc ) ;
(since C++23)
(constexpr since C++26)
forward_list ( std:: initializer_list < T > init,
const Allocator & alloc = Allocator ( ) ) ;
(11) (constexpr since C++26)

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

1) The default constructor. Constructs an empty forward_list with a default-constructed allocator.
If Allocator is not DefaultConstructible , the behavior is undefined.
2) Constructs an empty forward_list with the given allocator alloc .
3) Constructs a forward_list with count default-inserted objects of T . No copies are made.
If T is not DefaultInsertable into forward_list , the behavior is undefined.
4) Constructs a forward_list with count copies of elements with value value .
If T is not CopyInsertable into forward_list , the behavior is undefined.
5) Constructs a forward_list with the contents of the range [ first , last ) . Each iterator in [ first , last ) is dereferenced exactly once.
This overload participates in overload resolution only if InputIt satisfies the requirements of LegacyInputIterator .
If T is not EmplaceConstructible into forward_list from * first , the behavior is undefined.
6) Constructs a forward_list with the contents of the range rg . Each iterator in rg is dereferenced exactly once.
If T is not EmplaceConstructible into forward_list from * ranges:: begin ( rg ) , the behavior is undefined.
7) The copy constructor. Constructs a forward_list with the contents of other . The allocator is obtained as if by calling std:: allocator_traits < Allocator > :: select_on_container_copy_construction
( other. get_allocator ( ) ) .
8) The move constructor. Constructs a forward_list with the contents of other . The allocator is obtained by move construction from other. get_allocator ( ) .
9) Same as the copy constructor, except that alloc is used as the allocator.
If T is not CopyInsertable into forward_list , the behavior is undefined.
10) Same as the move constructor, except that alloc is used as the allocator.
If T is not MoveInsertable into forward_list , the behavior is undefined.
11) Equivalent to forward_list ( il. begin ( ) , il. end ( ) , alloc ) .

Contents

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 pair of iterators defining the source range of elements to copy
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
rg - a container compatible range

Complexity

1,2) Constant.
3,4) Linear in count .
5) Linear in std:: distance ( first, last ) .
6) Linear in ranges:: distance ( rg ) .
7) Linear in other. size ( ) .
8) Constant.
9) Linear in other. size ( ) .
10) Linear in other. size ( ) if alloc ! = other. get_allocator ( ) , otherwise constant.
11) Linear in init. size ( ) .

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.reqmts]/67 , and a more direct guarantee is under consideration via LWG issue 2321 .

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overload ( 6 )

Example

Run this code 
#include <forward_list>
#include <iostream>
#include <string>
template<typename T>
std::ostream& operator<<(std::ostream& s, const std::forward_list<T>& v)
{
    s.put('{');
    for (char comma[]{'\0', ' ', '\0'}; const auto& e : v)
        s << comma << e, comma[0] = ',';
    return s << "}\n";
}
int main()
{
    // C++11 initializer list syntax:
    std::forward_list<std::string> words1{"the", "frogurt", "is", "also", "cursed"};
    std::cout << "1: " << words1;
    // words2 == words1
    std::forward_list<std::string> words2(words1.begin(), words1.end());
    std::cout << "2: " << words2;
    // words3 == words1
    std::forward_list<std::string> words3(words1);
    std::cout << "3: " << words3;
    // words4 is {"Mo", "Mo", "Mo", "Mo", "Mo"}
    std::forward_list<std::string> words4(5, "Mo");
    std::cout << "4: " << words4;
    const auto rg = {"cat", "cow", "crow"};
#ifdef __cpp_lib_containers_ranges
    std::forward_list<std::string> words5(std::from_range, rg); // overload (6)
#else
    std::forward_list<std::string> words5(rg.begin(), rg.end()); // overload (5)
#endif
    std::cout << "5: " << words5;
}

Output: 

1: {the, frogurt, is, also, cursed}
2: {the, frogurt, is, also, cursed}
3: {the, frogurt, is, also, cursed}
4: {Mo, Mo, Mo, Mo, Mo}
5: {cat, cow, crow}

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 was explicit made non-explicit
LWG 2210 C++11 overload ( 3 ) did not have an allocator parameter added the parameter
N3346 C++11 for overload ( 3 ) , the elements in
the container were value-initialized 		they are default-inserted

See also

assigns values to the container
(public member function)
assigns values to the container
(public member function)