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How is inheritance implemented at the memory level?

Suppose I have

class A           { public: void print(){cout<<"A"; }};
class B: public A { public: void print(){cout<<"B"; }};
class C: public A {                                  };

How is inheritance implemented at the memory level?

Does C copy print() code to itself or does it have a pointer to the it that points somewhere in A part of the code?

How does the same thing happen when we override the previous definition, for example in B (at the memory level)?

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Moeb Avatar asked Apr 21 '10 04:04

Moeb


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1 Answers

Compilers are allowed to implement this however they choose. But they generally follow CFront's old implementation.

For classes/objects without inheritance

Consider:

#include <iostream>

class A {
    void foo()
    {
        std::cout << "foo\n";
    }

    static int bar()
    {
        return 42;
    }
};

A a;
a.foo();
A::bar();

The compiler changes those last three lines into something similar to:

struct A a = <compiler-generated constructor>;
A_foo(a); // the "a" parameter is the "this" pointer, there are not objects as far as
          // assembly code is concerned, instead member functions (i.e., methods) are
          // simply functions that take a hidden this pointer

A_bar();  // since bar() is static, there is no need to pass the this pointer

Once upon a time I would have guessed that this was handled with pointers-to-functions in each A object created. However, that approach would mean that every A object would contain identical information (pointer to the same function) which would waste a lot of space. It's easy enough for the compiler to take care of these details.

For classes/objects with non-virtual inheritance

Of course, that wasn't really what you asked. But we can extend this to inheritance, and it's what you'd expect:

class B : public A {
    void blarg()
    {
        // who knows, something goes here
    }

    int bar()
    {
        return 5;
    }
};

B b;
b.blarg();
b.foo();
b.bar();

The compiler turns the last four lines into something like:

struct B b = <compiler-generated constructor>
B_blarg(b);
A_foo(b.A_portion_of_object);
B_bar(b);

Notes on virtual methods

Things get a little trickier when you talk about virtual methods. In that case, each class gets a class-specific array of pointers-to-functions, one such pointer for each virtual function. This array is called the vtable ("virtual table"), and each object created has a pointer to the relevant vtable. Calls to virtual functions are resolved by looking up the correct function to call in the vtable.

like image 190
Max Lybbert Avatar answered Nov 01 '22 06:11

Max Lybbert