[CPP]Default members and conversion

Default member functions

Automatically generated functions

• 4 functions created by default if needed
• You can make them unavailable(e.g. private)
• Or change their behaviour
• 1. A default constructor (no parameters needed)
• 1. A copy constructor(copy one object to another)
• 1. An assignment operator ( = operator)
• 1. A destructor

A default constructor

• Automatically created if and only if you do not create any other constructors
• This is why you can still create objects even when classes appear to have no constructors
• If you create any constructor, compiler will not create a default one for you.
• Since C++ 11 you can tell it to create a default one anyway by putting “= default” instead of “{}”
• ClassName() = default;
• 
• if we comment on “DemoClass() = default;” This program will not compile.

The copy constructor

The copy constructor is used to initialize one object from another of the same type
This includes when a copy is implicitly made:

• Passing object as a parameter into a function
• Returning object by value from a function

A copy constructor is created by default, unless you create your own.

• Takes a constant reference to the object to copy from
• Has to be a reference!(to avoid needing to copy)
• If not a reference then copying parameter value means copying the object
  

Assignment operator

• Used when value of one object is assigned to another
• Assignment operator will be created by default
• Takes a reference to the one we are getting values from
• Returns a reference to *this, so we can chain these: ob1= ob2 = ob3;
  
  Above is a every interesting code:
• We create demoClass1 use default constructor, so demo1 will print “1”
• We create demo2 using constructor, will print “2”
• We make a copy constructor on demo3, it will add i to 1, so will print “3”
• We create demo4 and using assignment operator, so will print “12”
• then we create demo5, 6 and 7
• We chain them, we first look at the right hand side, demo7 is “6”, so demo6 is “16” and demo5 will be “26”

Destructor

• A destructor is created if you do not create one yourself
• Default destructor does nothing
• 

Conversion constructors

A conversion constructor is a constructor with one parameter.
It is a normal constructor but with single parameter, so we can pass this parameter when initializing:

Keyword “explicit”

• Providing a one-parameter constructor provides a conversion constructor
• This allows compiler to use it to convert to the type whenever it wants/needs to do so
• To avoid this, use the keyword explicit
• Then you CAN NOT USE = parameter to initialize
  

Conversion operator

• Convert from a class into something else(like int, float)
• Uses operator overloading syntax
•