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CS253 Member Initialization

The Old Way

class Name {
  public:
    Name() { first="John"; last="Doe"; }
    Name(string f) { first=f; last="Doe"; }
    Name(string f, string l) { first=f; last=l; }
    string full() const { return first + " " + last; }

  private:
    string first, last;
};

Name a, b("Cher"), c("Barack", "Obama");
cout << a.full() << '\n' << b.full() << '\n'
     << c.full() << '\n';

John Doe
Cher Doe
Barack Obama

Member Initialization

Member initialization:

class Name {
  public:
    Name() : first("John"), last("Doe") { }
    Name(string f) : first(f), last("Doe") { }
    Name(string f, string l) : first(f), last(l) { }
    string full() const { return first + " " + last; }

  private:
    string first, last;
};

Name a, b("Cher"), c("Barack", "Obama");
cout << a.full() << '\n' << b.full() << '\n'
     << c.full() << '\n';

John Doe
Cher Doe
Barack Obama

Delegation

BAD!

class Name {
  public:
    Name() { Name("John", "Doe"); }
    Name(string f) { Name(f, "Doe"); }
    Name(string f, string l) : first(f), last(l) { }
    string full() const { return first + " " + last; }

  private:
    string first, last;
};

Name a, b("Cher"), c("Barack", "Obama");
cout << a.full() << '\n' << b.full() << '\n'
     << c.full() << '\n';

 
 
Barack Obama

Delegation

Good:

class Name {
  public:
    Name() : Name("John", "Doe") { }
    Name(string f) : Name(f, "Doe") {}
    Name(string f, string l) : first(f), last(l) { }
    string full() const { return first + " " + last; }

  private:
    string first, last;
};

Name a, b("Cher"), c("Barack", "Obama");
cout << a.full() << '\n' << b.full() << '\n'
     << c.full() << '\n';

John Doe
Cher Doe
Barack Obama

Details

• When should you use member initialization?
  • When you have a non-default initial value for your member variable.
  • When you want to emphasize the default initial value.
  • When the member variable is expensive to construct, and you have a non-default initial value to put in it.
• When must you use member initialization?
  • When the member variable is only initializable, and not assignable:
    • a const variable
    • a reference variable
    • an object without a default constructor
    • initializing a base class
• Member variables are constructed in declaration order, not member initialization list order.

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