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Chapter 13. Copy Control

Exercise 13.1:

What is a copy constructor? When is it used?

A copy constructor is a constructor which first parameter is a reference to the class type and any additional parameters have default values.

When copy initialization happens and that copy initialization requires either the copy constructor or the move constructor.

  • Define variables using an =
  • Pass an object as an argument to a parameter of nonreference type
  • Return an object from a function that has a nonreference return type
  • Brace initialize the elements in an array or the members of an aggregate class
  • Some class types also use copy initialization for the objects they allocate.

Exercise 13.2:

Explain why the following declaration is illegal:

Sales_data::Sales_data(Sales_data rhs);

If declaration like that, the call would never succeed to call the copy constructor, Sales_data rhs is an argument to a parameter, thus, we'd need to use the copy constructor to copy the argument, but to copy the argument, we'd need to call the copy constructor, and so on indefinitely.

Exercise 13.3:

What happens when we copy a StrBlob? What about StrBlobPtrs?

// added a public member function to StrBlob and StrBlobPrts
long count() {
    return data.use_count(); // and wptr.use_count();
}

// test codes in main()
StrBlob str({"hello", "world"});
std::cout << "before: " << str.count() << std::endl; // 1
StrBlob str_cp(str);
std::cout << "after: " << str.count() << std::endl;  // 2

ConstStrBlobPtr p(str);
std::cout << "before: " << p.count() << std::endl; // 2
ConstStrBlobPtr p_cp(p);
std::cout << "after: " << p.count() << std::endl; // 2

when we copy a StrBlob, the shared_ptr member's use_count add one.

when we copy a StrBlobPrts, the weak_ptr member's use_count isn't changed.(cause the count belongs to shared_ptr)

Exercise 13.4:

Assuming Point is a class type with a public copy constructor, identify each use of the copy constructor in this program fragment:

Point global;
Point foo_bar(Point arg) // 1. Pass an object as an argument to a parameter of nonreference type
{
    Point local = arg, *heap = new Point(global); // 2.3. copy initialization
    *heap = local; // 4. Define variables using an `=`
    Point pa[ 4 ] = { local, *heap }; // 5. Brace initialize the elements in an array
    return *heap; // 6. Return an object from a function that has a nonreference return type
}

Exercise 13.5

Exercise 13.6:

What is a copy-assignment operator? When is this operator used? What does the synthesized copy-assignment operator do? When is it synthesized?

The copy-assignment operator is function named operator=.

This operator is used when assignment occurred.

The synthesized copy-assignment operator assigns each nonstatic member of the right-hand object to corresponding member of the left-hand object using the copy-assignment operator for the type of that member.

It is synthesized when the class does not define its own.

Exercise 13.7:

What happens when we assign one StrBlob to another? What about StrBlobPtrs?

In both cases, shallow copy will happen. All pointers point to the same address. The use_count changed the same as 13.3.

Exercise 13.8

Exercise 13.9:

What is a destructor? What does the synthesized destructor do? When is a destructor synthesized?

The destructor is a member function with the name of the class prefixed by a tilde(~).

As with the copy constructor and the copy-assignment operator, for some classes, the synthesized destructor is defined to disallow objects of the type from being destoryed. Otherwise, the synthesized destructor has an empty function body.

The compiler defines a synthesized destructor for any class that does not define its own destructor.

Exercise 13.10:

What happens when a StrBlob object is destroyed? What about a StrBlobPtr?

When a StrBlob object destroyed, the use_count of the dynamic object will decrement. It will be freed if no shared_ptr to that dynamic object.

When a StrBlobPter object is destroyed the object dynamicaly allocated will not be freed.

Exercise 13.11

Exercise 13.12:

How many destructor calls occur in the following code fragment?

bool fcn(const Sales_data *trans, Sales_data accum)
{
    Sales_data item1(*trans), item2(accum);
    return item1.isbn() != item2.isbn();
}

3 times. There are accum, item1 and item2.

Exercise 13.13

Exercise 13.14:

Assume that numbered is a class with a default constructor that generates a unique serial number for each object, which is stored in a data member named mysn. Assuming numbered uses the synthesized copy- control members and given the following function:

void f (numbered s) { cout << s.mysn << endl; }

what output does the following code produce?

numbered a, b = a, c = b;
f(a); f(b); f(c);

Three identical numbers.

Exercise 13.15:

Assume numbered has a copy constructor that generates a new serial number. Does that change the output of the calls in the previous exercise? If so, why? What output gets generated?

Yes, the output will change. cause we don't use the synthesized copy-control members rather than own defined.The output will be three different numbers.

Exercise 13.16:

What if the parameter in f were const numbered&? Does that change the output? If so, why? What output gets generated?

Yes, the output will change. cause the function f haven't any copy operators. Thus, the output are the same when pass the each object to f.

Exercise 13.17