Ctor initailzers and other related topics.

Objectives

Notes

• Default parameter values
  • It is permissible to provide default values to a function parameter
  • It is done by type paramID = value
  • It must be done from the last parameter to the first.
    • Trailing arguments
    • Parameters are filled in from the start
    • With any missing replaced by default parameters.
    • You place the values either in the function prototype or the implementation but not both.
      • I prefer the prototype as this is the "documentation" of the function.
    • If you provide defaults to all parameters of a constructor then this becomes the default constructor.
    • See paramDemo.cpp.
  • Ctor Initializers
    • These are needed
      • So we can call base class constructors with parameters.
      • Or any component constructors that are parameterized.
      • We wish to be efficient
    • Three ways to provide a value to class member data
      • Inside the constructor
      • In the class declaration (ie where the variables are declared)
      • In the constructor initialzer
    • In the class declaration
      • This is static, ie it is set for the class and can not be changed without recompiling.
      • It is the lowest default, ie used if the other two are not.
    • The constructor
      • This is dynamic in that it can be based on parameters or calculated at run time.
      • It is, however a copy operation, not an initialization
      • this possibly consumes more time.
      • And is not acceptable with the creating an object that does not have a default constructor.
    • The ctor initializer
      • This overrides from the definition
      • But will be overridden by the constructor body
      • These can be based on parameter values or other computations, so are dynamic
    • ctor-initializer format

      Identifier(parameter list): member{value}, member{value}, ... { body} 

    • Note: the members must be initialized in the order they are declared
  • Ctor-initializers support two additional class variable member types
    • Constant values, that can be initialzied at run time.
    • Reference member variables.
  • Reference member variables
    • I don't know if I like this or not, but remember, a reference points to memory.
    • This is fine if the memory is guaranteed to exist as long as the reference is in lifetime.
    • The reference as a class member makes this somewhat more challenging.
    • But the same would be true of a pointer member, so this is just a tool we need to be careful with.
    • A reference means we don't copy a large data structure, just point to it.
    • This wold be useful in a game, the game board is always a problem for me.
  • With both const members and reference members you need a copy constructor if you are going to copy.
  • And I don't think you can do an assignment operator to cover these fields, they will get the default value.
    • But I think it forces you to write one anyway.
  • Take a look at ctorDemo.cpp