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Showing content from https://cplusplus.com/reference/map/map/emplace_hint/ below:

public member function

<map>

std::map::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 map if its key is unique, 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).

The insertion only takes place if no other element in the container has a key equivalent to the one being emplaced (elements in a map container are unique).

If inserted, 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.

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 map container (the elements in a map 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 If the function successfully inserts the element (because no equivalent element existed already in the map), the function returns an iterator to the newly inserted element.

Otherwise, it returns an iterator to the equivalent element within the container.

Member type iterator is a bidirectional iterator type that points to an element.

Example

// map::emplace_hint
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> mymap;
  auto it = mymap.end();

  it = mymap.emplace_hint(it,'b',10);
  mymap.emplace_hint(it,'a',12);
  mymap.emplace_hint(mymap.end(),'c',14);

  std::cout << "mymap contains:";
  for (auto& x: mymap)
    std::cout << " [" << x.first << ':' << x.second << ']';
  std::cout << '\n';

  return 0;
}

Output:

mymap contains: [a:12] [b:10] [c:14]

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 safetyStrong 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.

See also

map::emplace: Construct and insert element (public member function)

map::insert: Insert elements (public member function)

map::erase: Erase elements (public member function)

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