Object Life Cycles: Constructors

• All objects are constructed via a constructor.
  • These are sometimes called ctors.
  • If none is provided, the compiler will write one for you that does noting.
• A constructor that takes no parameters is called the Default Constructor
  • It is frequently beneficial to have such a constructor.
  • You need one for building objects without arguments.
    • Both cases below require a default constructor.

    • class ItemT {
      };
      
      vector<ItemT> items(10); 
      ItemT data[10]; 

  • We are warned that this code is incorrect but will probably compile

    • class ItemT{
          ItemT();
          ...
      };
      
      ...
      ItemT foo(); 

    • The problem is that the compiler thinks you are declaring a function.
    • ItemT myItem{}; is the proper way to call the default constructor.
    • ItemT myItem; is acceptable as well.
• If you don't want to make a default constructor but want one explicitly you can write

  ItemT() = default; 

• If you don't want any constructor you can delete it explicitly

  ItemT() = delete; 

• Constructors can be overloaded.
  • There must be a difference in one of the following
    • Parameters, type or count
    • Modifiers, but probably not for a constructor.
• Look at constructor.cpp
• constructor initializers
  • Classes are composed of other objects.
  • These objects are initialized BEFORE the constructor is called.
  • If we want to initialize these objects properly we can use a constructor initializer (or ctor-initializer)
  • We had to do this in the case of the exception
    • We called the base class constructor.
  • Or when we have a member object that does not have a default constructor.
  • In other cases, it is more efficient
    • Construct an object properly
    • Vs Construct an object then call transformers (setters)
  • Name(params): data1(value), data2{value}, ... { // function def}
  • This also works with basic types.
• WARNING: Constructors are called in the order the objects are declared, not the order listed in the constructor initalizer.
• See ctorInit.cpp
• Parameters only show up in the class definition or in the function definition if you put it in two files.

  • // .h file
    foo(int a =7);
    bar(int a);
    
    // .cpp
    foo(int a)
    bar(int a = 7);
             

• Parameters must be given default values from right to left.