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)
explicit forward_list ( const Allocator & alloc ) ;
(2)
explicit forward_list ( size_type count,
const Allocator & alloc = Allocator ( ) ) ;
(3)
forward_list ( size_type count, const T & value,
const Allocator & alloc = Allocator ( ) ) ;
(4)
template < class InputIt >

forward_list ( InputIt first, InputIt last,

const Allocator & alloc = Allocator ( ) ) ;
(5)
template < container-compatible-range < T > R >

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

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

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 std:: forward_list < T > , the behavior is undefined.
4) Constructs a forward_list with count copies of elements with value value .
If T is not CopyInsertable into std:: forward_list < T > , 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 std:: forward_list < T > 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 std:: forward_list < T > from * ranges:: begin ( rg ) , the behavior is undefined.
7-10) Constructs a forward_list with the contents of other .
7) The copy constructor. The allocator is obtained as if by calling std:: allocator_traits < Allocator > ::
select_on_container_copy_construction
( other. get_allocator ( ) ) .
8) The move constructor. 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 std:: forward_list < T > , the behavior is undefined.
10) Same as the move constructor, except that alloc is used as the allocator.
If T is not MoveInsertable into std:: forward_list < T > , the behavior is undefined.
11) Equivalent to forward_list ( il. begin ( ) , il. end ( ) , alloc ) .

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
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)
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