[C++] Copy Control (part 1)

Copy, Assign, and Destroy

When we define a class, we specify what happens when objects of the class are copied, moved, assigned, and destroyed. A class controls these operations by defining five special member functions: copy contructor, copy-assignment contructor, move constructor, move-assignment operator and destructor.

The copy and copy-assignement define what happen when an object is initialized from another object of the same type.

The move and move-assignment define what happens when we assign an object of a class to another object of the same type.

The destructor define what happens when an object of the type cease to exist.

We refer to these operation as copy control

If a class does not define all of the copy-control members, compiler automatically define the missing operations.

A contructor is a copy contrucotr if the first parameter is a reference to the class type and additional parameters provide default value.

class Foo{
public:
    Foo();    　　　　　　// default constructor
    Foo(Const Foo&);    // copy constructor
};

When we do not define a copy constructor, compiler synthesize it for us. Unlike the default constructor, a copy construcotr is synthesized even if we define other constructor.

    string dots(, '.');    // direct constructor
    string s2 = dots;        // copy constructor
    string null_book = "999-999-999";    // copy constructor

Copy initialization happens not only when we define a variable using an =, but also when we:

• pass an obejct as a argument to a parameter of a nonreference type
• Return an object from an function that has a nonreference return type
• Brace initialize the elements in an array or the members of an aggregate class.

    Sales_data trans, accm;
    trans = accm;    // use the Sales_data copy assignment

class Foo{
public:
    Foo& operator=(const Foo&);    // assignment operation
    //...
};

Assignment operators ordinarily should return a reference to their left-hand operand.

Just as does for the copy constructor, the compiler generates synthesize copy assignment operator for a class if we do not define its own.

Generally, it assigns each members of the right-hand object to the corresponding members of the left-hand object using the copy-assignment operator of the type of the member.

// equals to the synthesized copy-assignment operator
Sales_data & Sales_data::operator=(const Sales_data &rhs){
    bookNo = rhs.bookNo;    // call the string::operator=
    units_sold = rhs.units_sold;    // use the built-in int assignment
    revenue = rhs.revenue;
    return *this;
};

The Destructor is a member function with the same name of the type prefixed by the tilde(~). It has no return value and no parameter; there is always only one destructor for a given class.

class Foo{
    ~Foo();    // destructor
};

Unlike ordinarily pointers, the smart pointers are class types and have destructor.

{ // new scope
    // p and p2 point to dynamically allocated objects
    Sales_data *p = new Sales_data;
    auto *p2 = make_shared<Sales_data> ();    // p2 is a shared_prt
    Sales_data item(*p2);    // copy constructor copies *p into item
    vector<Sales_data> vec;    // local variable
    vec.push_back(*p2);    // copy the object to which p2 points
 
    delete p;    // destructor called pointed to by p
}    // exit local scope; destructor called on item, p2, vec
    // destroy p2 decreases its use count; if the count goes to 0, the object is freeed
    // destroying vec destroy the elements in vec.

The only memory our code has to manage is the object we directly allocated. Our code frees only the dynamically allocated object bound to p.

The destructor is not run when a reference or pointer to a object go out of scope.

Generally, the synthesized destructor has an empty function body.

class Sales_data{
public:
    ~Sales_data() { }
    // ...
}

If the class need a destrutor, it almost surely need the copy-assignment operator and copy construcotr.

Consider what would happen if we give HasPrt a destructor but use the synthesized copy-assignment operator and copy constructor.

class HasPtr{
public:
    HasPrt(const string & s = string()) : ps(new string(s)), i() { }
    ~HasPrt() { delete ps; }
    // WRONG: HasPtr need a copy constructor and copy-assignment
    // ...
private:
    string * ps;
};

The copy constructor and copy-assignment operator copy pointer member, meaning that multiple objects may be pointing to the same memory:

HasPtr f(HasPtr hp){
    HasPtr ret = hp;    // copy the given HasPtr
    return ret;    // ret and hp are detroyed
}

When f returns, both hp and ret are destroyed and HasPtr destructor run on each of these objects. These objects contain the same pointer value. This code will delete the pointer twice, this is an error.

    HasPtr p("some value");
    f(p);    // When f complete, the memory to which ps points is freed
    HasPtr q(p);    // now both p and q points to invalid memory

The second rule of thumb: If a class needs a copy constructor, it also almost surely needs a copy-assignement operator, and vice versa.

Nevertheless, needing either a copy constructor or a copy-assignment operator, does not indicate the need of destructor.

A deleted function is one that be declared but may not be used in any other way.

We can prevent copies by defining the copy constructor and copy-assignment operator as deleted  functions, marked as =deleted.

struct NoCopy{
    NoCopy = default;
    NoCopy(const NoCopy &) = delete;    // no copy constructor
    NoCopy & operator = (const NoCopy &) = delete;    // no copy-assignment operator
    ~NoCopy() = delete;
    // other members
}

Although the primary use of deleted function is to suppress the control members, deleted functions are sometimes useful when we guide to function-matching process.

The Destructor should a deleted member.

We can dynamically allocate objects with deleted destructor, but we cannot free them.

It is not possible to define an object or delete a pointer of an object of a type with a deleted destructor.

If a class has a data member that cannot be default constructed, copied, assigned, destroyed, then its corresponding member will be a deleted function.

In essense, the copy-control members are synthesized as deleted function when it is impossible to copy, assign, or destroy a member of this class.

It should not be surprising that a class with a const members cannot use the synthesized copy-assignment construtor: after all, the operator attempt to assign to every member. It is not possible to assign a new value to const member.

If the copy-assignment operator is synthesized for a class with a reference member, the left-hand operand will continue to refer to the same object as it did before the assignment. This behaviour is unlikely to be desired. The synthesized copy-assignment operator is defined as deleted if the class has a reference member.

copy Control and Resource Management

In general, we have two choices: we can define copy operations to make a class behave like a value or like a pointer.

• Classes that behave like a value have their own state. The copy and original are independent.
• Classes that behave like a pointer share state. the copy and original use the same underlying data.

The IO types and unique_ptr do not allow copying or assignment, so they provide neither valuelike or pointerlike behavour.

What we do when we copy a pointer member determeines whether a class like HasPtr has valuelike or pointerlike behavours.

Classes that Act like Values

To provide valuelike behavour, each object has to have a copy of resources that the class manages. That means each objects of HasPtr has to have a copy of string to which ps points. To implement HasPtr needs:

• A copy constructor that copy the string, not just the pointer.
• A destructor to free the string
• A copy-assignment operator to free the existing string of left-hand operand, and copy the string from right-hand operand.

class HasPtr{
public:
    HasPtr(const string & s= string()) : ps(new string(s)), i() { }
    HasPtr(const HasPtr & p): ps(new string(*p.ps)), i(p.i) { }
    HasPtr & operator=(const HasPtr &);
    ~HasPtr() { delete ps; }
private:
    string *ps;
    int i;
};

Our code simple enough that we've define all but the copy-assignment operator in the class body.

Most copy-assignment should work with the destructor and copy constructor.

Valuelike Copy-Assignment Operator. For copy-assignment, it is crucially important that actions are done in sequence that is correct even if a class assign to itself.

HasPtr & HasPtr::operator=(const HasPtr & rhs){
    auto newP = new string(rhs.ps);    // copy the underlying string
    delete ps;        // free the old memory
    ps = newp;
    i = rhs.i;
    return *this;
};

Classes That Act Like Pointers

The easiest way to make a class like a pointer is to use share_ptrs to manage the resources in the classes.

However, sometimes we want to manage resources directly. In such case, it is useful to use a reference count. We will redefine HasPtr to provide pointerlike behavour, and we will do our won reference counting.

How can the copy and the orignal points to the same counter?

• The way to do that is to store the counter in dynamicac memory.
• Another way is to store the counter on a static member of the class (By Tony)

class HasPtr{
public:
    // constructor allocates a new string and a new counter, which is set to 1
    HasPtr(count string &s = string()): ps(new string(s)), i(), use(new size_t()) { }
    // copy constructor copies all data members, and increase the counter
    HasPtr(const HasPtr& p): ps(p.ps), i(p.i), use(p.use) { use++; }
    HasPtr& operator=(const HasPtr);
    ~HasPtr();
private:
    string *Ps;
    int i;
    size_t *use;    // member to keep trace of how many objects share *ps
};
 
HasPtr::~HasPtr(){
    --*use;
    if(*use == ){
        delete ps;
        delete use;
    }
}

HasPtr& HasPtr::Operator=(const HasPtr &rhs){
    ++*rhs.use;
    --*use;
    if(*use == ){
        delete ps;
        delete use;
    }
    ps = rhs.ps;
    i = rhs.i;
    use = rhs.use;
    return *this;
}

Reference:

C++ Primer, Fifth Edition, chapter 13 Copy Control