How does C++ store functions and objects in memory? [duplicate]

Question

Lets say we have a class

class A
{
    int x;
public:
    void sayHi()
    {
        cout<<"Hi";
    }
};

int main()
{
    A *a=NULL;
    a->sayHi();
}

The above code will compile on Turbo C (where I tested) and print Hi as output.

I was expecting crash because a is NULL. More over if I make sayHi() function virtual, it says

Abnormal temination(Segmentation fault in gcc) 

I know a lot of it is implementation dependent but if anybody could throw some light on any implementation or just give an overview it would be really nice.

Answer 1

As a generalisation, the layout of a object instantiated from a class with no super classes and virtual functions is as follows:

* - v_ptr  ---> *  pTypeInfo
|               |- pVirtualFuncA
|               |- pVirtualFuncB
|- MemberVariableA
|- MemberVariableB

v_ptr is a pointer to the v-table - which contains addresses of virtual functions and RTTI data for the object. Classes without virtual functions don't have v-tables.

In your example above, class A has no virtual methods and thus no v-table. This means the implementation of sayHi() to call can be determined at compile time and is invariant.

The compiler generates code that sets the implicit this pointer to a and then jumps to the beginning of sayHi(). Since the implementation has no need for the object's contents, the fact that it works when the pointer is NULL is a happy coincidence.

If you were to make sayHi() virtual, the compiler cannot determine the implementation to call at compiler time, so instead generates code that looks up the address of the function in the v-table and calls it. In your example where a is NULL, the compiler reads the contents of address 0, causing the abort.

Answer 2

In C++, the methods of a class are not stored inside the instances of that class. They're simply some "special" functions that transparently accept the this pointer in addition to the arguments specified by the programmer.

In your case, the sayHi() method does not reference any of the class fields, therefore, the this pointer (which is NULL) is never followed.

Make no mistake, though, this is still undefined behavior. Your program may choose to send nasty emails to your contact list when you invoke this. In this particular instance, it does the worst thing, and appears to work.

The virtual method case has been added since I answered the question, but I won't refine my answer, since it's included by others' answers.