A constructor of a class is mainly responsibly for turning the raw memory allotted to an object into a usable object.
DEFAULT CONSTRUCTOR:
The default constructor is a constructor that takes no argument.
Part::Part()
String() and String(const char* str) can be combined into a single constructor that has a default argument: String(const char* str=0)

for every class there can be only one default constructor.

COPY CONSTRUCTOR:
A copy constructor is a special constructor that can be called to copy an object. The copy constructor for class Part has the form
Part::Part(const Part&). 

DESTRUCTOR:
Whenever an object goes out of scope it is destroyed and the memory used by that particular object will be reclaimed. 
Part::~Part().