what is noexcept in c++ code example

Example: noexcept c++

#include <iostream>
#include <utility>
#include <vector>
 
void may_throw();
void no_throw() noexcept;
auto lmay_throw = []{};
auto lno_throw = []() noexcept {};
class T{
public:
  ~T(){} // dtor prevents move ctor
         // copy ctor is noexcept
};
class U{
public:
  ~U(){} // dtor prevents move ctor
         // copy ctor is noexcept(false)
  std::vector<int> v;
};
class V{
public:
  std::vector<int> v;
};
 
int main()
{
 T t;
 U u;
 V v;
 
 std::cout << std::boolalpha
           << "Is may_throw() noexcept? " << noexcept(may_throw()) << '\n'
           << "Is no_throw() noexcept? " << noexcept(no_throw()) << '\n'
           << "Is lmay_throw() noexcept? " << noexcept(lmay_throw()) << '\n'
           << "Is lno_throw() noexcept? " << noexcept(lno_throw()) << '\n'
           << "Is ~T() noexcept? " << noexcept(std::declval<T>().~T()) << '\n'
           // note: the following tests also require that ~T() is noexcept because
           // the expression within noexcept constructs and destroys a temporary
           << "Is T(rvalue T) noexcept? " << noexcept(T(std::declval<T>())) << '\n'
           << "Is T(lvalue T) noexcept? " << noexcept(T(t)) << '\n'
           << "Is U(rvalue U) noexcept? " << noexcept(U(std::declval<U>())) << '\n'
           << "Is U(lvalue U) noexcept? " << noexcept(U(u)) << '\n'  
           << "Is V(rvalue V) noexcept? " << noexcept(V(std::declval<V>())) << '\n'
           << "Is V(lvalue V) noexcept? " << noexcept(V(v)) << '\n';  
}