C sharp Constructor

Question

When an object is created and call is made to the constructor what exactly is going on so that I have a great understanding? Need an explanation that will stick. Any ideas? namespace EX12_1 { public class Fraction { private int numerator; private int denominator; public Fraction(int numerator int denominator) { this.numerator = numerator; this.denominator = denominator; } } public class Test { public void Run() { Fraction firstFraction = new Fraction(3 4); Console.WriteLine(
firstFraction: {0} firstFraction.ToString()); } } static void Main() { Test t = new Test(); t.Run(); } }

Vulpes · Answer

The static operator '+' takes two existing Fraction instances and returns a new Fraction instance that is the sum of the existing instances. Notice that the two 'return' statements in the operator method both call the Fraction constructor in order to do this An operator is in effect a static method which looks something like this: public static Fraction Plus(Fraction lhs Fraction rhs) { // code } and a statement such as this: Fraction f3 = f1 + f2; is transformed into this: Fraction f3 = Fraction.Plus(f1 f2);

F D · Answer

namespace EX12_1 { public class Fraction { private int numerator; private int denominator; public Fraction(int numerator int denominator) { this.numerator = numerator; this.denominator = denominator; } } public static Fraction operator+(Fraction lhs Fraction rhs) { if (lhs.denominator == rhs.denominator) { return new Fraction(lhs.numerator + rhs.numerator lhs.denominator); } int firstProduct = lhs.numerator * rhs.denominator; int secondProduct = rhs.numerator * rhs.denominator; return new Fraction(firstProduct + secondProduct lhs.denominator * rhs.denominator); } public class Test { public void Run() { Fraction firstFraction = new Fraction(3 4); Console.WriteLine(
firstFraction: {0} firstFraction.ToString()); Fraction f2 = new Fraction(2 4); Console.WriteLine(f2: {0} f2.ToString()); Fraction f3 = f1 + f2;//Why is it that when this statement is executed the Class Fraction f3 is created after the public static Fraction operator + function is executed first? } } static void Main() { Test t = new Test(); t.Run(); } }

F D · Answer

That is really helpful. Thank you so much.

Vulpes · Answer

There are two other points I should mention for completeness. Before any code in the constructor runs it first calls its base class constructor. This in turn calls its base class constructor and so on up to System.Object's constructor which is the ultimate base class of all classes in .NET. In this case Fraction's base class is System.Object (as no other class is inherited) and so technically the latter's constructor is called before the fields are initialized. However System.Object's constructor contains no code and therefore does nothing. So it's likely that this step will actually be 'optimized away' by the just-in-time compiler. The overhead I mentioned (8 bytes on a 32-bit system or 16 bytes on a 64-bit system) consists of two components: a pointer to the class's 'method table' and a 'sync block' index. Sync blocks are used to 'lock' an object. Only one thread can lock an object at any one time which makes these useful for synchronization in multithreading program. The sync block index is a pointer to one of these sync blocks which are kept in an array on the heap.

Vulpes · Answer

When a call is made to the Fraction constructor sufficient memory is allocated on the heap to contain Fraction's two int fields plus some overhead and all bytes are initially set to zero. The two fields then have their values set using the arguments numerator and denominator passed to the constructor. A reference to the new Fraction object (in effect a pointer to where the object is stored) is then returned and assigned to the local variable firstFraction. This reference enables its ToString() method to be called.