C++11: Overloaded Constructors and Delegating Constructors

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Section 6.17 showed how to overload functions. A class’s constructors and member functions can also be overloaded. Overloaded constructors typically allow objects to be initialized with different types and/or numbers of arguments. To overload a constructor, provide in the class definition a prototype for each version of the constructor, and provide a separate constructor definition for each overloaded version. This also applies to the class’s member functions.

In Figs. 9.4–9.6, the Time constructor with three parameters had a default argument for each parameter. We could have defined that constructor instead as four overloaded constructors with the following prototypes:

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Time(); // default hour, minute and second to 0
Time( int ); // initialize hour; default minute and second to 0
Time( int, int ); // initialize hour and minute; default second to 0
Time( int, int, int ); // initialize hour, minute and second

Just as a constructor can call a class’s other member functions to perform tasks, C++11 now allows constructors to call other constructors in the same class. The calling constructor is known as a delegating constructor—it delegates its work to another constructor. This is useful when overloaded constructors have common code that previously would have been defined in a private utility function and called by all the constructors.

The first three of the four Time constructors declared above can delegate work to one with three int arguments, passing 0 as the default value for the extra parameters. To do so, you use a member initializer with the name of the class as follows:

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Time::Time()
   : Time( 0, 0, 0 ) // delegate to Time( int, int, int )
{
} // end constructor with no arguments
Time::Time( int hour )
   : Time( hour, 0, 0 ) // delegate to Time( int, int, int )
{
} // end constructor with one argument
Time::Time( int hour, int minute )
   : Time( hour, minute, 0 ) // delegate to Time( int, int, int )
{
} // end constructor with two arguments

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