map::emplace_hint

Artikel
07/29/2015

Inserts an element constructed in place (no copy or move operations are performed), with a placement hint.

template<class... Args>
   iterator emplace_hint(
      const_iterator where,
      Args&&... args);

Parameters

Parameter	Description
args	The arguments forwarded to construct an element to be inserted into the map unless the map already contains that element or, more generally, unless it already contains an element whose key is equivalently ordered.
where	The place to start searching for the correct point of insertion. (If that point immediately precedes where, insertion can occur in amortized constant time instead of logarithmic time.)

Return Value

An iterator to the newly inserted element.

If the insertion failed because the element already exists, returns an iterator to the existing element with its key.

Remarks

No iterators or references are invalidated by this function.

During emplacement, if an exception is thrown, the container's state is not modified.

The value_type of an element is a pair, so that the value of an element will be an ordered pair with the first component equal to the key value and the second component equal to the data value of the element.

Example

// map_emplace.cpp
// compile with: /EHsc
#include <map>
#include <string>
#include <iostream>

using namespace std;

template <typename M> void print(const M& m) {
    cout << m.size() << " elements: " << endl;

    for (const auto& p : m) {
        cout << "(" << p.first <<  "," << p.second << ") ";
    }

    cout << endl;
}

int main()
{
    map<string, string> m1;

    // Emplace some test data
    m1.emplace("Anna", "Accounting");
    m1.emplace("Bob", "Accounting");
    m1.emplace("Carmine", "Engineering");

    cout << "map starting data: ";
    print(m1);
    cout << endl;

    // Emplace with hint
    // m1.end() should be the "next" element after this emplacement
    m1.emplace_hint(m1.end(), "Doug", "Engineering");

    cout << "map modified, now contains ";
    print(m1);
    cout << endl;
}

Output

map starting data: 3 elements:
(Anna,Accounting) (Bob,Accounting) (Carmine,Engineering)

map modified, now contains 4 elements:
(Anna,Accounting) (Bob,Accounting) (Carmine,Engineering) (Doug,Engineering)

Requirements

Header: <map>

Namespace: std

Freigeben über