std::priority_queue<T,Container,Compare>::priority_queue

#include <complex>
#include <functional>
#include <iostream>
#include <queue>
#include <vector>
 
int main()
{
 std::priority_queue <int> pq1;
 pq1.push(5);
 std::cout << "pq1.size() = " << pq1.size() << '\n';
 
 std::priority_queue <int> pq2 {pq1};
 std::cout << "pq2.size() = " << pq2.size() << '\n';
 
 std::vector <int> vec {3, 1, 4, 1, 5};
 std::priority_queue <int> pq3 {std::less <int>(), vec};
 std::cout << "pq3.size() = " << pq3.size() << '\n';
 
 for (std::cout << "pq3 : "; !pq3.empty(); pq3.pop())
 std::cout << pq3.top() << ' ';
 std::cout << '\n';
 
 // Demo With Custom Comparator:
 
 using my_value_t = std::complex <double>;
 using my_container_t = std::vector <my_value_t>;
 
 auto my_comp = [](const my_value_t& z1, const my_value_t& z2)
 {
 return z2.real() < z1.real();
 };
 
 std::priority_queue <my_value_t,
 my_container_t,
 decltype(my_comp)> pq4{my_comp};
 
 using namespace std::complex_literals;
 pq4.push(5.0 + 1i);
 pq4.push(3.0 + 2i);
 pq4.push(7.0 + 3i);
 
 for (; !pq4.empty(); pq4.pop())
 {
 const auto& z = pq4.top();
 std::cout << "pq4.top() = " << z << '\n';
 }
 
 // TODO: C++23 range-aware ctors
}

pq1.size() = 1
pq2.size() = 1
pq3.size() = 5
pq3 : 5 4 3 1 1
pq4.top() = (3,2)
pq4.top() = (5,1)
pq4.top() = (7,3)