NAME¶

std::list::merge - std::list::merge

Synopsis¶

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

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 operator<.
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
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);
comp -
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
list<T,&nbsp;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 <list>

std::ostream& operator<<(std::ostream& ostr, const std::list<int>& list)
{
for (const int i : list)
ostr << ' ' << i;
return ostr;
}

int main()
{
std::list<int> list1 = {5, 9, 1, 3, 3};
std::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 300 C++98 the effect when *this and other refer specified as no-op
to the same object was not specified
LWG 1207 C++98 it was unclear whether iterators and/or keep valid
references will be invalidated
O(1) node moving could not be guaranteed if the behavior is
LWG 1215 C++98 get_allocator() != other.get_allocator() undefined in this
case

See also¶

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