C++ Object Memory Model

Summary

    // some examples:
    struct a { void foo(); };           // no need for virtual table
    struct b : a { virtual foo1(); };   // need vtable, and vptr
    struct c : b { void bar(); };       // no extra virtual table, 1 vptr (b) suffices
    struct d : b { virtual bar(); };    // 1 vtable, d extends b's vtable

    struct e : d, b {};                 // 2 vptr, 1 for the d and 1 for b
    struct f : virtual b {};            // 1 vptr, f reuse b's vptr to locate subobj b
    struct g : virtual b {};            // 1 vptr, g reuse b's vptr to locate subobj b
    struct h : f, g {};                 // 2 vptr, 1 for f, 1 for g
                                        // h can locate subobject b using f's vptr

Basically each sub-object of a type that requires its own dynamic dispatch (cannot directly reuse the parents) would need its own virtual table and vptr.

Class with inheritance

class Base {
public:
    int baseVar;
};

class Derived : public Base {
public:
    double derivedVar;
    virtual void foo();
};

Memory Layout

Base Object

+---------------------+ 
| baseVar  | 
+---------------------+

Derived Object

+---------------------+ 
| vptr | // Pointing to Derived vTable
+---------------------+ 
| baseVar  |  // from Base 
+---------------------+ 
| derivedVar  |  //from Derived 
+---------------------+

Class with inheritance (Base is also virtual)

class Base {
public:
    int baseVar;
    virtual void foo();
};

class Derived : public Base {
public:
    double derivedVar;
    virtual void foo();
};

Memory Layout

Base Object

+---------------------+ 
| vptr | // Pointing to vTable for Base
+---------------------+ 
| baseVar  | 
+---------------------+

Derived Object

+---------------------+ 
| vptr | // Pointing to Derived vTable
+---------------------+ 
| baseVar  |  // from Base 
+---------------------+ 
| derivedVar  |  //from Derived 
+---------------------+

Class with multiple inheritance

class Base1 {
public:
    int baseVar1;
    virtual void foo();
};

class Base2 {
public:
    int baseVar2;
    virtual void foo();
};

class Derived : public Base, public Base2 {
public:
    double derivedVar;
    void foo() override;
};

Memory Layout

Base Object1

+---------------------+ 
| vptr | // Pointing to vTable for Base1
+---------------------+ 
| baseVar1  | 
+---------------------+

Base Object2

+---------------------+ 
| vptr | // Pointing to vTable for Base2
+---------------------+ 
| baseVar2  | 
+---------------------+

Derived Object

+---------------------+ 
| vptr | // Pointing to Derived vTable for Base1 (Base1 + Derived methods)
+---------------------+ 
| baseVar  | 
+---------------------+ 
| vptr | // Pointing to Derived vTable for Base2
+---------------------+ 
| baseVar2 | 
+---------------------+ 
| derivedVar  |  //from Derived 
+---------------------+

References

https://shaharmike.com/cpp/vtable-part1/
https://stackoverflow.com/questions/3342035/how-many-vptr-will-a-object-of-classuses-single-multiple-inheritance-have