emplace vs insert in C++ STL

In C++, all containers (vector, stack, queue, set, map, etc) support both insert and emplace operations.

Both are used to add an element in the container.
The advantage of emplace is, it does in-place insertion and avoids an unnecessary copy of object. For primitive data types, it does not matter which one we use. But for objects, use of emplace() is preferred for efficiency reasons.

CPP

// C++ code to demonstrate difference between
// emplace and insert
#include<bits/stdc++.h>
using namespace std;

int main()
{
    // declaring map
    multiset<pair<char, int>> ms;

    // using emplace() to insert pair in-place
    ms.emplace('a', 24);

    // Below line would not compile
    // ms.insert('b', 25);

    // using insert() to insert pair in-place
    ms.insert(make_pair('b', 25));

    // printing the multiset
    for (auto it = ms.begin(); it != ms.end(); ++it)
        cout << " " << (*it).first << " "
             << (*it).second << endl;

    return 0;
}

Output: 

 a 24
 b 25

Time Complexity: The time complexity depends upon the type of the container. Both the operations have same time complexity.

Vector: O(1)

Priority Queue: O(log n)

Set: O(log n)

map: O(log n)

Please refer Inserting elements in std::map (insert, emplace and operator []) for details.