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CS253 Inherited Methods

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Simple Example

class Foo {
  public:
    Foo(const string &n) : name(n) { }
    string name;
};

Foo a("Diane"), b(a);
cout << a.name << ' ' << b.name << '\n';

Diane Diane

There’s no copy ctor, so who set b.name to Diane?

Default Copy Ctor

class Foo {
  public:
    Foo(const string &n) : name(n) { }
    string name;
};

Foo a("Diane"), b(a);
cout << a.name << ' ' << b.name << '\n';

Diane Diane

There is a default copy ctor, which performs a bitwise copy on the object.

No

NO!

I Lied

The default copy ctor does not do a bitwise copy.
The default copy ctor does a memberwise copy.
You can’t just bitwise copy a C++ object.
What if it contains pointers to dynamically-allocated data?
A bitwise copy is a shallow copy; we almost always want a deep copy.
Do a memberwise copy by copying each member datum.

Like this

The default copy ctor & assignment operators look like this:

class Foo {
  public:
    Foo(const string &n) : name(n) { }
    Foo(const Foo &rhs) : name(rhs.name) { }
    Foo &operator=(const Foo &rhs) {
        name = rhs.name;
        return *this;
    }
    string name;
};

Foo a("Diane"), b(a);
b = a;
cout << a.name << ' ' << b.name << '\n';

Diane Diane

Missing

If you write any constructor, then the system will not provide a default constructor.
However, you still get the default copy ctor and assignment operator.
Don’t depend on defaults; make your intentions clear:

class Foo {
  public:
    Foo() = default;                    // I love this one.
    Foo &Foo(const Foo &) = delete;     // Not wild about this one.
    ...
};

Inheritance

Great! That explains everything!

Wait—what about inheritance?

Inheritance Example

struct Base {   // struct ≡ public access
    string name;
};
struct Derived : public Base {
    Derived(const string &n) { name = n; }
};

Derived a("Dorothy"), b(a);
cout << a.name << ' ' << b.name << '\n';

Dorothy Dorothy

How did that work‽ Who copied name? Sure, the Base default copy ctor would copy name, but nobody called that.

Sure Did

In fact, the Derived copy ctor called the Base copy ctor.
The base class is, in some ways, considered to be a member of the derived class called “Base”.
Therefore, the default copy ctor & assignment operators also copy the base class.

Code with Default Methods

struct Base {   // struct ≡ public access
    Base() { }
    Base(const Base &rhs) : name(rhs.name) { }
    Base &operator=(const Base &rhs) {
        name = rhs.name;
        return *this;
    }
    string name;
};
struct Derived : public Base {
    Derived(const string &n) { name = n; }
    Derived(const Derived &rhs) : Base(rhs) { }
    Derived &operator=(const Derived &rhs) {
        Base::operator=(rhs);
        return *this;
    }
};

Derived a("Dorothy"), b(a);
cout << a.name << ' ' << b.name << '\n';

Dorothy Dorothy

CS253: Software Development with C++

Spring 2019

Inherited Methods