public member function
<map>
std::multimap::emplace_hint
template <class... Args>
iterator emplace_hint (const_iterator position, Args&&... args);
Construct and insert element with hint
Inserts a new element in the multimap, with a hint on the insertion position. This new element is constructed in place using args as the arguments for the construction of a value_type (which is an object of a pair type).
This effectively increases the container size by one.
The value in position is used as a hint on the insertion point. The element will nevertheless be inserted at its corresponding position following the order described by its internal comparison object, but this hint is used by the function to begin its search for the insertion point, speeding up the process considerably when the actual insertion point is either position or close to it.
The element is constructed in-place by calling allocator_traits::construct with args forwarded.
The relative ordering of equivalent elements is preserved, and newly inserted elements follow their equivalents already in the container.
Parameters
- position
- Hint for the position where the element can be inserted.
The function optimizes its insertion time if position points to the element that will follow the inserted element (or to the end, if it would be the last).
Notice that this does not force the new element to be in that position within the multimap container (the elements in a multimap always follow a specific order).
const_iterator is a member type, defined as a bidirectional iterator type that points to elements.
- args
Arguments used to construct a new object of the mapped type for the inserted element.
Arguments forwarded to construct the new element (of type
pair<const key_type, mapped_type>).
This can be one of:
- Two arguments: one for the
key, the other for the
mapped value.
- A single argument of a
pair type with a value for the
key as first member, and a value for the
mapped value as second.
-
piecewise_construct as first argument, and two additional arguments with
tuples to be forwarded as arguments for the
key value and for the
mapped value respectivelly.
See
pair::pair for more info.
Return value
An iterator to the newly inserted element.
Member type iterator is a bidirectional iterator type that points to an element.
Example
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
|
// multimap::emplace_hint
#include <iostream>
#include <string>
#include <map>
int main ()
{
std::multimap<std::string,int> mymultimap;
auto it = mymultimap.end();
it = mymultimap.emplace_hint(it,"foo",10);
mymultimap.emplace_hint(it,"bar",20);
mymultimap.emplace_hint(mymultimap.end(),"foo",30);
std::cout << "mymultimap contains:";
for (auto& x: mymultimap)
std::cout << " [" << x.first << ':' << x.second << ']';
std::cout << '\n';
return 0;
}
| |
Output:
mymultimap contains: [bar:20] [foo:10] [foo:30]
|
Complexity
Generally, logarithmic in the container size.
Amortized constant if the insertion point for the element is position.
Iterator validity
No changes.
Data races
The container is modified.
Concurrently accessing existing elements is safe, although iterating ranges in the container is not.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the container.
If allocator_traits::construct is not supported with the appropriate arguments, it causes undefined behavior.