473,326 Members | 2,182 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes and contribute your articles to a community of 473,326 developers and data experts.

The sizeof an empty class or structure is 1

Banfa
9,065 Expert Mod 8TB
I thought I would write a little article about this because it is regularly asked as a question in the Forums.

Firstly this is a C++ issue, it is strictly against the rules of C to create a class with no members. This makes sense because the only real use for a structure or class with no data members and virtual functions is as the base from which to derive other classes and structures or as a container for non-virtual methods.

The reason this happens boils down to properly implementing the standard, one of the things the C++ standard says is that "no object shall have the same address in memory as any other variable". There is a very good reason for this, take an array of type T then subtracting the pointer to 1 entry in the array from another should give the difference in indexes

Expand|Select|Wrap|Line Numbers
  1. T array[5];
  2.  
  3. int diff = &array[3] - &array[2];
  4.  
  5. // diff = 1
  6.  
This is actually pointer arithmetic, we have taken 2 pointers and found their difference. The compiler performs this calculation using a formula equivalent to this

diff = ((char *)&array[3] - (char *)&array[2]) / sizeof T;

This is just an example, but this type of calculation reliant on the sizeof T occurs in all sorts of pointer arithmetic.

If objects were allowed to have the same address because their type was 0 sized then this calculation would not be possible because

Expand|Select|Wrap|Line Numbers
  1. &array[3] - &array[2] = &array[3] - &array[1]
  2.                       = &array[3] - &array[1]
  3.                       = &array[3] - &array[0]
  4.                       = 0
  5.  
It is not possible to distinguish the different objects in pointer arithmetic.

Additionally there is the problem that the compiler had to divided by sizeof T, which would be a divide by zero error, although this could be coded round.

Allowing objects to have the same memory address would result in the need to implement some rather complex code to handle pointer arithmetic on them.

So objects must not have the same memory address, that is they must be individually addressable. What is the easiest way to ensure this? Make sure that all types have a non-zero size. In order to achieve this the compiler adds a dummy byte to structures and classes that have no data members and no virtual functions so that they have a size of 1 rather than a size of 0 and then they are guaranteed to have a unique memory address.
Jun 11 '07 #1
5 35656
weaknessforcats
9,208 Expert Mod 8TB
A class (or struct) can have member functions without using any data members. Unless you can create an object of this class, you can't call the methods. For this reason, a class (or struct) that has no data members is created with a sizeof 1 byte.

This also allows you to define a pointer to an object of that class. You can't define a pointer of you cannot acquire the address of an object of the pointer type. One byte is the smallest addressable memory unit. The only time in C++ when you cannot create an object of the class is when it contains (or has inherited) a pure virtual function.

Expand|Select|Wrap|Line Numbers
  1. class MyClass
  2. {
  3.     public:
  4.        void AMethod() {cout << "Hello" << endl;}
  5. };
  6. int main()
  7. {
  8.     MyClass obj;
  9.     MyClass* ptr = &obj;
  10.     obj.AMethod();
  11.     ptr->AMethod();
  12.     cout << sizeof(obj) << endl;    //displays 1
  13. }
  14.  
When a class contains no data variables but does have at least one virtual function, the object is created containing the address of the virtual funciton table (VTBL) as part of the object. Because of this, the size of the object will now be 4.

Expand|Select|Wrap|Line Numbers
  1. class MyClass
  2. {
  3.     public:
  4.        virtual void AMethod() {cout << "Hello" << endl;}
  5. };
  6. int main()
  7. {
  8.     MyClass obj;
  9.     MyClass* ptr = &obj;
  10.     obj.AMethod();
  11.     ptr->AMethod();
  12.     cout << sizeof(obj) << endl;    //displays 4
  13. }
  14.  
Dec 10 '07 #2
I don't understand about the statement.

Its space overhead is one pointer in each object of a class with a virtual function plus one vtbl for each such class.

Please clear my doubt.

Thanks.
Mar 2 '08 #3
weaknessforcats
9,208 Expert Mod 8TB
I think you almost understand this.

There is one VTBL for all objects of a class with virtual functions.

Each object of the class has a pointer to that VTBL. Therefore, the overhead in each object is one pointer.
Mar 4 '08 #4
A class (or struct) can have member functions without using any data members. Unless you can create an object of this class, you can't call the methods. For this reason, a class (or struct) that has no data members is created with a sizeof 1 byte.

This also allows you to define a pointer to an object of that class. You can't define a pointer of you cannot acquire the address of an object of the pointer type. One byte is the smallest addressable memory unit. The only time in C++ when you cannot create an object of the class is when it contains (or has inherited) a pure virtual function.

Expand|Select|Wrap|Line Numbers
  1. class MyClass
  2. {
  3.     public:
  4.        void AMethod() {cout << "Hello" << endl;}
  5. };
  6. int main()
  7. {
  8.     MyClass obj;
  9.     MyClass* ptr = &obj;
  10.     obj.AMethod();
  11.     ptr->AMethod();
  12.     cout << sizeof(obj) << endl;    //displays 1
  13. }
  14.  
When a class contains no data variables but does have at least one virtual function, the object is created containing the address of the virtual funciton table (VTBL) as part of the object. Because of this, the size of the object will now be 4.

Expand|Select|Wrap|Line Numbers
  1. class MyClass
  2. {
  3.     public:
  4.        virtual void AMethod() {cout << "Hello" << endl;}
  5. };
  6. int main()
  7. {
  8.     MyClass obj;
  9.     MyClass* ptr = &obj;
  10.     obj.AMethod();
  11.     ptr->AMethod();
  12.     cout << sizeof(obj) << endl;    //displays 4
  13. }
  14.  
I understood that in case of virtual functions the size of the object is increasing by the VPtr (virtual pointer) which is pointing to base address of VTBLE, but please explain why it's 4 bytes exactly? why not some other value.
Oct 1 '08 #5
weaknessforcats
9,208 Expert Mod 8TB
I understood that in case of virtual functions the size of the object is increasing by the VPtr (virtual pointer) which is pointing to base address of VTBLE, but please explain why it's 4 bytes exactly? why not some other value.
It's the sizeof a memory address in a 32-bit operating system.
Oct 4 '08 #6

Sign in to post your reply or Sign up for a free account.

Similar topics

1
by: Sean W. Quinn | last post by:
Hey folks, I have a question regarding file handling, and the preservation of class structure. I have a class (and I will post snippets of code later in the post) with both primitive data...
2
by: Xiangliang Meng | last post by:
Hi, all. What will we get from sizeof(a class without data members and virtual functions)? For example: class abnormity { public: string name() { return "abnormity"; }
4
by: Sweety | last post by:
hello, why sizeof an empty class why 1 byte ? plz illustrate this ? bye
8
by: ishekara | last post by:
Hi all, An object of a class A which has no member variables and no member functions, still the size of the object is 1 byte, if there is a byte member variable then also the size of the object...
6
by: free2cric | last post by:
Hi, What will the syntax sound like for checking size of an empty class? what will be the size. Thanks, Cric
4
by: avranju | last post by:
Hi, We have a few exception marker classes in a project that are used only to throw exceptions. I mean stuff like this (this may not be such a hot idea for doing exception handling - a simple...
7
by: dost | last post by:
can any budy tell me why the size of the empty class object is 1 thank you kapil kaushik
19
by: Abubakar | last post by:
Hi, lets say I have the following class: class AAA{}; if I do: sizeof(AAA) it gives me 1. Why is the size 1 ? Regards,
2
by: Harish | last post by:
This was an interview question: The size of an empty class is 1 byte. Why? Should it not be 4 bytes? Because an invisible pointer "this" is part of every class?
6
by: deepakvsoni | last post by:
what is the size of an class with not data members? ex: class A { }; also sizeof ? class A{ int f(){ cout<<"Hello";
0
by: ryjfgjl | last post by:
ExcelToDatabase: batch import excel into database automatically...
0
isladogs
by: isladogs | last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM). In this month's session, we are pleased to welcome back...
0
by: jfyes | last post by:
As a hardware engineer, after seeing that CEIWEI recently released a new tool for Modbus RTU Over TCP/UDP filtering and monitoring, I actively went to its official website to take a look. It turned...
0
by: ArrayDB | last post by:
The error message I've encountered is; ERROR:root:Error generating model response: exception: access violation writing 0x0000000000005140, which seems to be indicative of an access violation...
1
by: PapaRatzi | last post by:
Hello, I am teaching myself MS Access forms design and Visual Basic. I've created a table to capture a list of Top 30 singles and forms to capture new entries. The final step is a form (unbound)...
1
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
0
by: af34tf | last post by:
Hi Guys, I have a domain whose name is BytesLimited.com, and I want to sell it. Does anyone know about platforms that allow me to list my domain in auction for free. Thank you
0
by: Faith0G | last post by:
I am starting a new it consulting business and it's been a while since I setup a new website. Is wordpress still the best web based software for hosting a 5 page website? The webpages will be...
0
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome former...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.