exception handling problem

Hi, everyone, I'm learning <<thinking c++>> volume Two, and testing the
code below.

/////////////////////////////////////////////////////////////////////////////
//: C01:Wrapped.cpp
#include <iostream>
#include <cstddef>
using namespace std;

template<class T, int sz = 1> class PWrap {
    T* ptr;
public:
    class RangeError {}; // Exception class
    PWrap() {
        ptr = new T[sz];
        cout << "PWrap constructor" << endl;
    }
    ~PWrap() {
        delete[] ptr;
        cout << "PWrap destructor" << endl;
    }
    T& operator[](int i) throw(RangeError) {
        if(i >= 0 && i < sz) return ptr[i];
        throw RangeError();
    }
};

class Cat {
public:
    Cat() { cout << "Cat()" << endl; }
    ~Cat() { cout << "~Cat()" << endl; }
    void g() {}
};

class Dog {
public:
    void* operator new[](size_t) {
        cout << "Allocating a Dog" << endl;
        //throw 47; //*******************************NOTE
        return 0;
    }
    void operator delete[](void* p) {
        cout << "Deallocating a Dog" << endl;
        ::delete[](p);
    }
};

class UseResources {
    PWrap<Cat, 3> cats;
    PWrap<Dog> dog;
public:
    UseResources() {
        cout << "UseResources()" << endl;
    }
    ~UseResources() {
        cout << "~UseResources()" << endl;
    }
    void f() { cats[1].g(); }
};

int main() {
    try {
        UseResources ur;
    } catch(int) {
        cout << "inside handler" << endl;
    } catch(...) {
        cout << "inside catch(...)" << endl;
    }
}
/////////////////////////////////////////////////////////////////////////////////
In the UseResources class, It will create *Three cats* and *One dog*,
When I comment the "throw 47" line in the class Dog implementation.
it has the result output:
=================================
Cat()
Cat()
Cat()
PWrap constructor
Allocating a Dog
PWrap constructor
UseResources()
~UseResources()
Deallocating a Dog
PWrap destructor
~Cat()
~Cat()
~Cat()
PWrap destructor
==================================

But When I uncomment the "throw 47" line, the result will be:

=================================
Cat()
Cat()
Cat()
PWrap constructor
Allocating a Dog
~Cat()
~Cat()
~Cat()
PWrap destructor
inside handler
==================================

It seems that an exception happened when we are create the Dog object.

My Question:
When an exception occurred in the allocate a Dog object,
Why was *~UseResources()* not called?
Why was *~Cat()* called?

How does the "exception mechanism" guarantee that the allocated object
will be cleared.

Someone can give this a brief trick? Is it about call stack unwinding?
Thank you very much.