set::emplace

Article
07/29/2015

Inserts an element constructed in place (no copy or move operations are performed).

template<class... Args>
    pair<iterator, bool> emplace(
        Args&&... args
);

Parameters

Parameter	Description
args	The arguments forwarded to construct an element to be inserted into the set unless it already contains an element whose value is equivalently ordered.

Return Value

A pair whose bool component returns true if an insertion was made, and false if the map already contained an element whose value had an equivalent value in the ordering. The iterator component of the return value pair returns the address where a new element was inserted (if the bool component is true) or where the element was already located (if the bool component is false).

Remarks

No iterators or references are invalidated by this function.

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

Example

// set_emplace.cpp
// compile with: /EHsc
#include <set>
#include <string>
#include <iostream>

using namespace std;

template <typename S> void print(const S& s) {
    cout << s.size() << " elements: ";

    for (const auto& p : s) {
        cout << "(" << p << ") ";
    }

    cout << endl;
}

int main()
{
    set<string> s1;

    auto ret = s1.emplace("ten");

    if (!ret.second){
        cout << "Emplace failed, element with value \"ten\" already exists."
            << endl << "  The existing element is (" << *ret.first << ")"
            << endl;
        cout << "set not modified" << endl;
    }
    else{
        cout << "set modified, now contains ";
        print(s1);
    }
    cout << endl;

    ret = s1.emplace("ten");

    if (!ret.second){
        cout << "Emplace failed, element with value \"ten\" already exists."
            << endl << "  The existing element is (" << *ret.first << ")"
            << endl;
    }
    else{
        cout << "set modified, now contains ";
        print(s1);
    }
    cout << endl;
}

Output

set modified, now contains 1 elements: (ten)

Emplace failed, element with value "ten" already exists.
  The existing element is (ten)

Requirements

Header: <set>

Namespace: std

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