std::set

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <set>
#include <string_view>
 
template<typename T>
std::ostream & operator<<(std::ostream & out, const std::set<T>& set)
{
 if (set.empty())
 return out << "{}";
 out << "{ " << *set.begin();
 std::for_each (std::next (set.begin()), set.end(), [&out](const T& element)
 {
 out << ", " << element;
 });
 return out << " }";
}
 
int main()
{
 std::set<int> set{1, 5, 3};
 std::cout << set << '\n';
 
 set.insert(2);
 std::cout << set << '\n';
 
 set.erase(1);
 std::cout << set << "\n\n";
 
 std::set<int> keys{3, 4};
 for (int key : keys)
 {
 if (set.contains(key))
 std::cout << set << " does contain " << key << '\n';
 else
 std::cout << set << " doesn't contain " << key << '\n';
 }
 std::cout << '\n';
 
 std::string_view word = "element";
 std::set<char> characters(word.begin(), word.end());
 std::cout << "There are " << characters.size() << " unique characters in "
 << std::quoted (word) << ":\n" << characters << '\n';
}

{ 1, 3, 5 }
{ 1, 2, 3, 5 }
{ 2, 3, 5 }
 
{ 2, 3, 5 } does contain 3
{ 2, 3, 5 } doesn't contain 4
 
There are 5 unique characters in "element":
{ e, l, m, n, t }