suppose A is a base class and B is the derved class. Both
have a method foo which is defined as a virtual method in
the base class. You have a pointer of classs B and you
typecast it to A. Now when you call pointer->foo, which
method gets called?
The next part of the question is, how does the compiler
know which method to call?
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Answer Posted / rahul
the answer is that compiler does only the the type checking at compile time it checks only that the type of reference or the type of object is being assigned to the type is assignable or not(i.e parent child relationship or cast operation should be valid otherwise it will give you compile time error) and compiler conveys the message to the runtime that this operation is valid so no type checking is done at run time, at runtime function call depends upon the actual object being referred by the type but the existance of that function is checked in the type class if the function doesn't exist in the type class the function will not be called even though it exist in the class which is being instantiated by type(reference type)
for example A a = new B();
a.foo();
here we are calling the foo() method of class B(because object of class B is being instantiated)using class A reference now here when we compile this program the compiler checks for foo() method in class A if this method doesn't exist in class A compiler even will not compile your program
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