Why Does Calling a Non-Virtual Method on a NULL Pointer Not Result in Undefined Behavior?

Accessing Class Members on a NULL Pointer

Despite being declared as a pointer to an instance of type Foo, the foo variable is initialized to 0, making it a null pointer. This may seem counterintuitive, as accessing members of a null pointer is typically undefined behavior. However, certain circumstances allow this behavior.

Non-Virtual Method Invocation

The non-virtual method say_hi() is successfully invoked despite the null pointer because it is not a virtual method. Virtual methods require a valid object to determine which function to call based on the object's type. Non-virtual methods, on the other hand, have a predetermined function address that the compiler uses to generate a direct function call instruction.

Specifically, the compiler generates code equivalent to the following:

void Foo_say_hi(Foo* this);
Foo_say_hi(foo);

Since the say_hi() function does not reference the this pointer, it bypasses the need to dereference a potentially null pointer.

Object Allocation

The foo variable is a local variable within the main function, so it is allocated on the function's stack. As a pointer variable, the actual object it points to is not allocated here. The null value indicates that the pointer does not reference a valid object.

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