How C++ object keeps information about its member functions

Question

class A {
       public :
       void printSometext() {
       std::cout << "printing A" << std::endl;
       }
    };
class B {
       public : 
       void printSometext() {
       std::cout << "printing B" << std::endl;
       }
    };

int main() {
   A* a = new A();
   a->printSometext();
   return 0;
}

How C++ object keeps information about its member functions. Lets consider above code. When I call printSometext on object "a" how does it know what function to call and how it finds the right method. When printing the size of the object it prints the summing size of its member variable (+allignments). So please provide some internal information how the member function call takes place.

Thanks, Deimus

Answer 1

Well, that's an interesting question, but let me try to answer that in a very methodical manner !!!

let's say compiler has to resolve a call like this : *

a->someFunc();

*.

Now, compiler will go over following steps methodically.

1.) Firstly, compiler knows the declared type of variable a, so it will check whether the declared type of object a (lets call this, class A for time being), have a method with the name someFunc() and that it must be public. This method could either be declared in class A, or it could be a derived method from one of the base class(es) of class A, but it doesn't matter to compiler and it just checks for it's existence with it's access specifier being public.

• Needless to say any error in this step will invite a compiler error.

2.) Secondly, once the method is validated to be a part of the class A, compiler has to resolve the call to the correct method, since many methods could be there with same name (thanks to function overloading). This process of resolving correct method is called overloading resolution. Compiler achieves this by matching the signatures of the called method with all the overloaded methods that are part of the class. So, of all the someFunc() s only the correct someFunc() (matching the signatures with called method) will be found and considered further.

3.) Now comes the difficult part, It may very well happen that someFunc() may have been overridden in one of the subclasses of the class A (lets call this class AA and needless to say it is some subclass of A), and that variable a (declared to be of type A) may actually be referring to an object of class AA, (this is permissible in C++ to enable polymorphism). Now, if the someFunc() method is declared to be of type virtual, in base class (i.e. Class A) and someFunc() has been overriden by subclass(es) of A (either in AA or classes between A and AA), the correct version of someFunc() have to be found out by the compiler.

• Now, imagine you're the compiler and you have this task of finding whether the class AA has this method. Obviously, class AA will have this method since, it is a subclass of A and public access of A in class A has already been validated in Step 1 by the compiler !!! . But Alternatively, as mentioned in previous paragraph, someFunc() may be overriden by class AA (or any other class between A and AA), which is what compiler needs to catch. Therefore, you (since, your'e playing the compiler) could do a systematic check to find the bottom-most (lowest in the inheritance tree) overridden method someFunc() starting from class A and ending at class AA. In this search, you'll look for same method signatures as was validated in overloading resolution. This method will be the method which will be invoked.

• Now, you may be wondering, "What the heck", is this search done everytime ? ... Well, Not really. The compiler knows the overhead of finding this everytime, and as such, maintains a data-structure called Virtual Table for every class type. Think of virtual table, as mapping from method signatures (which are publicly accessible) to the function pointers. This virtual table is made by compiler during compilation process and is maintained in-memory during program execution. In our example, class A and class AA will both have their own virtual tables. And when compiler has to be find someFunc() in class AA (since actual object pointed by variable a is of type AA), it will simply find the function pointer through the virtual table of Class AA. This is as simple has hashing into the table and is a constant time operation.

Regards

AViD

Answer 2

You have got the basics of C++ programming wrong. a does not know about printSomeText at the runtime, it is the compiler and linker which translates above code to binary code which performs these tasks. At runtime, there is just a bunch of instructions.

Answer 3

Adding to the Asha's answer (+1):

If you really want to understand how stuff works internally, I suggest to learn assembly and explore the generated machine code. That's what I did.

Answer 4

The compiler knows the types of the variables. When you use the . or the -> operator, it looks up the symbol to the left in the context of the class type of the right hand argument. So in your example, it only finds A::printSomeText.

-- James Kanze