CODEWITHSUNDEEP
c++inheritancevirtual
Aishwarya Kumar Singh
Aishwarya Kumar Singh

Reputation: 67

Why size of classes is larger in case of virtual inheritance?

Virtual base class is a way of preventing multiple instances of a given class appearing in an inheritance hierarchy when using multiple inheritance . Then for the following classes

class level0 {
    int a;
    public :
    level0();
};

class level10:virtual public level0 {
    int b;
    public :
    level10();
};

class level11 :virtual public level0 {
    int c;
    public :
    level11();
};

class level2 :public level10,public level11 { 
    int d;
    public:
    level2();
};

I got following sizes of the classes

size of level0 4

size of level10 12

size of level11 12

size of level2 24

but when I removed virtual from inheritance of level10 and level11 I got following output

sizeof level0 4

sizeof level10 8

sizeof level11 8

sizeof level2 20

If virtual inheritance prevents multiple instances of a base class, then why size of classes is greater in case of virtual inheritance?

Upvotes: 5

Views: 372

Answers (2)

santosh kumar
santosh kumar

Reputation: 585

As you can see in this, it is very clear that if the virtual inheritance is used, the compiler adds one offset pointer to point base class rather than including the base class members in its own memory. That's the reason for increasing size. If it will be x64 bit m\c, pointer size will be 8. And the output will be followings

size of level0 4

size of level10 16

size of level11 16

size of level2 32

Upvotes: 0

John Dibling
John Dibling

Reputation: 101456

Because when using virtual inheritence, the compiler will create* a vtable to point to the correct offsets for the various classes, and a pointer to that vtable is stored along with the class.

  • "Will create" -- vtables are not dictated by the Standard, but the behaviors implied by virtual inheritence is. Most compilers use vtables to implement the functionality dictated by the Standard.

Upvotes: 5

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