C++ - 阐明何时以及如何调用析构函数

Question

So I have an entire program below that creates Book objects, initializes them, and prints any constructors/destructors that are created or destroyed throughout the execution of the program.

I have ran my code (and pasted the output below), and I am having trouble understanding how the destructors are being called. So I know that the constructors are destroyed in the opposite order in which they were created. But I don't get why four of the destructor statements have an id of 4. I'm assuming one came from the "explicit call to the copy constructor", the other came from "declaring and initializing book 6 from book 5", and the other came from the first part of "declaring and initializing books 1 to 4." But I'm confused as to where the extra id of 4 came from?

Additionally, I was wondering why a "-- dtor: 0" wasn't printed for the "declaring book 5" part where default ctor: 0 was created.

I would really appreciate any clarification!

main.cc:

#include <iostream>
#include <string>
using namespace std;

#include "Book.h"


void func1(Book);
void func2(Book&);


int main()
{
  cout<<endl<<"Declaring and initializing books 1 to 4..."<<endl;

  Book b1(1, "Ender's Game", "Orson Scott Card");
  Book b2(2, "Dune", "Frank Herbert");
  Book b3(3, "Foundation", "Isaac Asimov");
  Book b4(4, "Hitch Hiker's Guide to the Galaxy", "Douglas Adams");

  cout<<endl<<"Declaring book 5..."<<endl;
  Book b5;
  b5.print();

  cout<<endl<<"Assigning book 4 to 5..."<<endl;
  b5 = b4;
  b5.print();

  cout<<endl<<"Declaring and initializing book 6 from book 5..."<<endl;
  Book b6 = b5;
  b6.print(); 

  cout<<endl<<"Calling func1()..."<<endl;
  func1(b1);

  cout<<endl<<"Calling func2()..."<<endl;
  func2(b2);

  cout<<endl<<"Explicit call to copy constructor..."<<endl;
  Book b7(b6);


  cout << endl << endl;

  return 0;
}

void func1(Book b)
{
  b.print();
}

void func2(Book& b)
{
  b.print();
}

book.cc:

#include <iostream>
#include <string>
using namespace std;
#include "Book.h"

Book::Book(int i, string t, string a)
{
  cout<<"-- default ctor:  "<< i <<endl;
  id     = i;
  title  = t;
  author = a;
}

Book::Book(const Book& other)
{
  id     = other.id;
  title  = other.title;
  author = other.author;

  cout<<"-- copy ctor:  "<< id <<endl;
}


Book::~Book()
{
  cout<<"-- dtor:  "<< id <<endl;
}

void Book::print()
{
  cout<<"** "<< title <<" by "<<author<<endl;
}

book.h:

#ifndef BOOK_H
#define BOOK_H

#include <string>
using namespace std;

class Book
{
  public:
    Book(int=0, string="Unknown", string="Unknown");
    Book(const Book&);
    ~Book();
    void print();

  private:
    int id;
    string title;
    string author;
};

#endif

OUTPUT:

Declaring and initializing books 1 to 4...
-- default ctor:  1
-- default ctor:  2
-- default ctor:  3
-- default ctor:  4

Declaring book 5...
-- default ctor:  0
** Unknown by Unknown

Assigning book 4 to 5...
** Hitch Hiker's Guide to the Galaxy by Douglas Adams

Declaring and initializing book 6 from book 5...
-- copy ctor:  4
** Hitch Hiker's Guide to the Galaxy by Douglas Adams

Calling func1()...
-- copy ctor:  1
** Ender's Game by Orson Scott Card
-- dtor:  1

Calling func2()...
** Dune by Frank Herbert

Explicit call to copy constructor...
-- copy ctor:  4


-- dtor:  4
-- dtor:  4
-- dtor:  4
-- dtor:  4
-- dtor:  3
-- dtor:  2
-- dtor:  1

Answer 1

I don't get why four of the destructor statements have an id of 4

because you assign b4 to b5 in statement

b5 = b4;

then copy construct b6 = b5; and b7(b6); each of them having id = 4, so destructor prints

-- dtor: 4

Additionally, I was wondering why a "-- dtor: 0" wasn't printed for the "declaring book 5" part where default ctor: 0 was created.

because when Book b5; is created it had id = 0 but when code assigned b4 to b5 id became 4, hence no "-- dtor: 0" was printed.