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

 

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std::forward_list

Member functions
forward_list::forward_list forward_list::~forward_list forward_list::operator= forward_list::assign forward_list::assign_range (C++23)   forward_list::get_allocator Element access forward_list::front Iterators forward_list::before_beginforward_list::cbefore_begin forward_list::beginforward_list::cbegin forward_list::endforward_list::cend Capacity forward_list::empty forward_list::max_size	Modifiers forward_list::clear forward_list::emplace_front forward_list::push_front forward_list::insert_after forward_list::emplace_after forward_list::erase_after forward_list::insert_range_after (C++23) forward_list::prepend_range (C++23) forward_list::pop_front forward_list::resize forward_list::swap Operations forward_list::merge forward_list::splice_after forward_list::removeforward_list::remove_if forward_list::reverse forward_list::unique forward_list::sort
Non-member functions
operator==operator<=> (C++20) swap(std::forward_list) erase(std::forward_list)erase_if(std::forward_list) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides (C++17)

 

void merge( forward_list& other );	(1)	(since C++11)
void merge( forward_list&& other );	(2)	(since C++11)
template< class Compare > void merge( forward_list& other, Compare comp );	(3)	(since C++11)
template< class Compare > void merge( forward_list&& other, Compare comp );	(4)	(since C++11)

The function does nothing if other refers to the same object as *this.

Otherwise, merges other into *this. Both lists should be sorted. No elements are copied, and the container other becomes empty after the merge. This operation is stable: for equivalent elements in the two lists, the elements from *this always precede the elements from other, and the order of equivalent elements of *this and other does not change.

No iterators or references become invalidated. The pointers and references to the elements moved from *this, as well as the iterators referring to these elements, will refer to the same elements of *this, instead of other.

1,2) Elements are compared using std::less<T>()(until C++14)std::less<>()(since C++14).

3,4) Elements are compared using comp.

If *this or other is not sorted with respected to the corresponding comparator, or get_allocator() != other.get_allocator(), the behavior is undefined.

Parameters

other	-	another container to merge
comp	-	comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1& a, const Type2& b); While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that an object of type forward_list<T, Allocator>::const_iterator can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- Compare must meet the requirements of Compare.

Return value

(none)

Exceptions

If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee). Except if the exception comes from a comparison.

Complexity

If other refers to the same object as *this, no comparisons are performed.

Otherwise, given \(\scriptsize N\)N as std::distance(begin(), end()) and \(\scriptsize R\)R as std::distance(other.begin(), other.end()):

1,2) At most \(\scriptsize N+R-1\)N+R-1 comparisons using operator<.

3,4) At most \(\scriptsize N+R-1\)N+R-1 applications of the comparison function comp.

Example

Run this code

#include <iostream>
#include <forward_list>
 
std::ostream& operator<<(std::ostream& ostr, const std::forward_list<int>& list)
{
    for (const int i : list)
        ostr << ' ' << i;
    return ostr;
}
 
int main()
{
    std::forward_list<int> list1 = {5, 9, 1, 3, 3};
    std::forward_list<int> list2 = {8, 7, 2, 3, 4, 4};
 
    list1.sort();
    list2.sort();
    std::cout << "list1: " << list1 << '\n';
    std::cout << "list2: " << list2 << '\n';
 
    list1.merge(list2);
    std::cout << "merged:" << list1 << '\n';
}

Output:

list1:  1 3 3 5 9
list2:  2 3 4 4 7 8
merged: 1 2 3 3 3 4 4 5 7 8 9

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 2045	C++11	O(1) node moving could not be guaranteed if get_allocator() != other.get_allocator()	the behavior is undefined in this case
LWG 3088	C++11	the effect when *this and other refer to the same object was not specified operator< could misbehave for pointer elements	specified as no-op implementation-defined strict total order used

See also

splice_after	moves elements from another forward_list (public member function)
merge	merges two sorted ranges (function template)
inplace_merge	merges two ordered ranges in-place (function template)
ranges::merge (C++20)	merges two sorted ranges (niebloid)
ranges::inplace_merge (C++20)	merges two ordered ranges in-place (niebloid)