CODEWITHSUNDEEP
c++class
ShankRam
ShankRam

Reputation: 324

class : size of object of derived class

Say I have two classes A and B as shown:

class A {
    void function1() {
        cout<<"hello";
    }
    int member1; 
public:
    char member2;
};

class B : public A 
{
    int member3;
    void function2() {
        cout<<"Hello to you too";
    }
public:
    float member4;
};

Now, if I create an object of class B, what will be the size of the object? Will it be the same even if the inheritance from class A is private or protected?

Upvotes: 2

Views: 556

Answers (4)

Peter
Peter

Reputation: 36597

sizeof for all types in C++ (other than char types, for which sizeof yields 1 by definition) is implementation defined.

As a rule of thumb, you would not expect a derived class to be smaller than a base class. But the difference - if any - depends on the compiler, the alignment of members, how the compiler manages padding to ensure members of different sizes (and the class types) are aligned, etc etc.

I wouldn't normally expect access (public, protected, private) to directly affect size of a class type. The compiler might choose to store the members in a different order depending on their access, or it might not. So the size may or may not change, but different compilers may do things differently. In principle, however, there is no need for the compiler to change the size of a class type simply because of changing the access of some member(s) or the inheritance.

Upvotes: 2

user586399
user586399

Reputation:

The private, protected or public inheritance have nothing to do with size. They are only affecting visibility of base members to outside code.

Upvotes: 0

6502
6502

Reputation: 114461

It depends on the compiler as they've some degree of freedom in this. My wild untested guess is that for example on a common x86 system A should be as big as two integers (4 bytes) and B as the same plus two other integers (8 bytes total).

The reason is that CPUs like aligned data so an int + char class is going to be allocated as int + char + 3-byte-padding to make an array of these classes to have all ints aligned correctly to addresses that are a multiple of 4.

int and float are the same size on x86 so that's where the two extra ints come from for B.

Given that there are no virtual functions no other data is needed per instance as all dispatching is going to be decided at compile time.

Private/public/protected are just logical walls enforced at compile time and don't need any extra storage at run-time.

Upvotes: 0

Adi Levin
Adi Levin

Reputation: 5233

The size of the object of class B will be the size of an object of class A plus the amount of memory taken by the extra data members that class B adds. Methods do not count, so you can add as many methods as you want, without increasing the size of objects.

Upvotes: 0

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