Question

Let’s say we have a class that looks like this:

class A 
{ 
    public:
        int FuncA( int x );
        int FuncB( int y );

        int a;
        int b;
};

Now, I know that objects of this class will be laid out in memory with just the two ints. That is, if I make a vector of instances of class A, there will be two ints for one instance, then followed by two ints for the second instance etc. The objects are POD.

BUT let’s say the class looks like this:

class B
{ 
    public:
        int FuncA( int x );
        int FuncB( int y );
};

What do objects of this class look like in memory? If I fill a vector with instances of B… what’s in the vector? I’ve been told that non-virtual member functions are in the end compiled as free functions somewhere completely unrelated to the instances of the class in which they’re declared (virtual function are too, but the objects store a vtable with function pointers). That the access restrictions are merely at the semantic, “human” level. Only the data members of a class (and the vtable etc.) actually make up the memory structure of objects.

So again, what do objects of class B look like in memory? Is it some kind of placeholder value? Something has to be there, I can take the object’s address. It has to point to something. Whatever it is, is the compiler allowed to inline/optimize out these objects and treat the method calls as just normal free function calls? If I create a vector of these and call the same method on every object, can the compiler eliminate the vector and replace it with just a bunch of normal calls?

I’m just curious.

Answer 1

All objects in C++ are guaranteed to have a sizeof >= 1 so that each object will have a unique address.

I haven’t tried it, but I would guess that in your example, the compiler would allocate but not initialize 1 byte for each function object in the array/vector.