emplace vs insert in C++ STL

In C++ STL, both emplace and insert functions are used to add elements to containers such as vectors, lists, and maps. But, emplace is more efficient than insert as it avoids unnecessary copying of object and does the insertion more efficiently than insert operation. In this article, we will discuss the all the differences between emplace and insert with examples.

Insert in C++ STL

The insert function in C++ STL is used to add new elements to a container (ie, set, map, vector, list, etc.). Following is the syntax of insert function:

// insert into a pair
multiset<pair> set1;
set1.insert(make_pair(1, "Alice"));

// insert into a vector
vector<int> vec1;
vec1.insert(vec1.begin(), 10); // insert 10 at the beginning

In the above code, we have used the insert function to add elements to a multiset and a vector. Note that to insert a pair into a multiset, we have to make a pair using make_pair function. This will create a copy of the pair and insert it into the multiset.

Example

In the code below, we will create a multiset and a vector, and we will insert elements into them using the insert function.

#include <iostream>
#include <set>
#include <vector>

using namespace std;

int main() {
    // Create a multiset of pairs
    multiset<pair<int, string>> set1;

    // Insert a pair into the multiset
    set1.insert(make_pair(1, "Alice"));
    set1.insert(make_pair(2, "Bob"));

    // Display the elements in the multiset
    cout << "Multiset elements:" << endl;
    for (const auto& p : set1) {
        cout << p.first << ": " << p.second << endl;
    }

    // Create a vector
    vector<int> vec1;

    // Insert an element at the beginning of the vector
    vec1.insert(vec1.begin(), 10);
    vec1.insert(vec1.end(), 20); // Insert at the end

    // Display the elements in the vector
    cout << "Vector elements:" << endl;
    for (const auto& v : vec1) {
        cout << v << " ";
    }
    cout << endl;

    return 0;
}

The output of the above code will be:

Multiset elements:
1: Alice
2: Bob
Vector elements:
10 20

Emplace in C++ STL

The emplace function in C++ STL is used insert new elements into a container without creating a copy of the object. Meaning, it constructs the object in place. Hence, this is more efficient than the insert function. The syntax of emplace function is given below:

// Using emplace to insert into a multiset
multiset<pair<int, string>> set1;
set1.emplace(1, "Alice");
set1.emplace(2, "Bob");

// Using emplace to insert into a vector
vector<int> vec1;
vec1.emplace(vec1.begin(), 10); // insert 10 at the beginning

Now, you can understand the difference between insert and emplace. When you are using emplace, you do not need to create a pair (or any object) to insert into the container. You can directly pass the arguments to the emplace function, and it will construct the object in place.

Example

In the code below, we will create a multiset and a vector, and we will insert elements into them using the emplace function.

#include <iostream>
#include <set>
#include <vector>

using namespace std;

int main() {
    // Create a multiset of pairs
    multiset<pair<int, string>> set1;

    // Emplace a pair into the multiset
    set1.emplace(1, "Alice");
    set1.emplace(2, "Bob");

    // Display the elements in the multiset
    cout << "Multiset elements:" << endl;
    for (const auto& p : set1) {
        cout << p.first << ": " << p.second << endl;
    }

    // Create a vector
    vector<int> vec1;

    // Emplace an element at the beginning of the vector
    vec1.emplace(vec1.begin(), 10);
    vec1.emplace(vec1.end(), 20); // Emplace at the end

    // Display the elements in the vector
    cout << "Vector elements:" << endl;
    for (const auto& v : vec1) {
        cout << v << " ";
    }
    cout << endl;

    return 0;
}

The output of the above code will be:

Multiset elements:
1: Alice
2: Bob
Vector elements:
10 20

Difference between Emplace and Insert

Here are the key differences between emplace and insert in C++ STL:

set1.emplace(3, "Charlie");

set1.insert(make_pair(3, "Charlie"));