C++ set emplace
C++ set emplace method is employed to expand the set container by adding fresh elements directly into the container without copying or moving them.
The constructor of the element is invoked by providing the arguments args that are passed to this function.
Insertion occurs exclusively when the key is not already present.
Syntax
template <class.... Args>
pair<iterator, bool> emplace (Args&&... args); //since C++ 11
Parameter
Parameters: The parameters passed to create an element that will be added to the set.
Return value
The emplace method provides a pair of booleans to signal whether the insertion took place and furnishes an iterator pointing to the newly added element.
Complexity
Logarithmic in the container size.
Iterator validity
No changes.
Data Races
The container is modified.
Traversing through ranges within the container poses a risk, whereas accessing existing elements concurrently is considered safe.
Exception Safety
If an error is raised, the container remains unchanged.
Example 1
Let's explore a basic illustration of adding elements to a set:
#include <iostream>
#include <set>
using namespace std;
int main(void) {
set<char> m;
m.emplace('a');
m.emplace('b');
m.emplace('c');
m.emplace('d');
m.emplace('e');
cout << "Set contains following elements" << endl;
for (auto it = m.begin(); it != m.end(); ++it)
cout << *it<< ", ";
return 0;
}
Output:
Set contains following elements
a, b, c, d, e,
In the aforementioned example, it merely adds the element to the set m using the provided key-value pairs.
Example 2
Let's explore a basic example to insert the element and validate for duplicate keys:
#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)
In the scenario mentioned, items are added to the set, and if an attempt is made to add an item with the key "ten" that already exists, an error message indicating the duplication of the key "ten" will be shown.
Example 3
Let's explore a basic illustration to calculate the total of the provided elements:
#include <iostream>
#include <set>
using namespace std;
int main()
{
// sum variable declaration
int sum = 0;
// set declaration
set<int> myset{};
myset.emplace(1);
myset.emplace(7);
myset.emplace(4);
myset.emplace(8);
myset.emplace(2);
myset.emplace(5);
myset.emplace(3);
// iterator declaration
set<int>::iterator it;
// finding sum of elements
while (!myset.empty()) {
it = myset.begin();
sum = sum + *it;
myset.erase(it);
}
// printing the sum
cout << "Sum of elements is: "<<sum;
return 0;
}
Output:
Sum of elements is: 30
Example 4
Let's see a simple example to insert the element:
#include <iostream>
#include <set>
#include <string>
using namespace std;
int main() {
typedef set<string> city;
string name;
city fmly ;
int n;
cout<<"Enter the number of family members :";
cin>>n;
cout<<"Enter the name of each member: \n";
for(int i =0; i<n; i++)
{
cin>> name; // Get key
fmly.emplace(name);
}
cout<<"\nTotal member of family is:"<< fmly.size();
cout<<"\nDetails of family members: \n";
cout<<"\nName \n ________________________\n";
city::iterator p;
for(p = fmly.begin(); p!=fmly.end(); p++)
{
cout<<(*p)<<" \n ";
}
return 0;
}
Output:
Enter the number of family members: 3
Enter the name of each member:
Bob
Robin
David
Total member of family is: 3
Details of family members:
Name
________________________
Bob
David
Robin
In the example provided, it merely adds the elements based on the user's selection.
