Size of a Empty class

This is a discussion on Size of a Empty class within the C++ Programming forums, part of the General Programming Boards category; Hi, Can anybody explain why size of a Empty class is 1 byte?...

  1. #1
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    Size of a Empty class

    Hi,
    Can anybody explain why size of a Empty class is 1 byte?

  2. #2
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    Because if the size is zero, how else could you make this [silly] example code work?
    Code:
    class c 
    {
    };
    
    c arr[10];
    c *p;
    ...
    if (p != &arr[0])  ...
    If the size is zero, then all c elements would have the same address.

    What difference does it make to you?

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  3. #3
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    It actually comes down to the fact that the unit in the C memory model is the char (which has sizeof 1 by definition), and that n*sizeof(anything) is the size of an array of n anything's. That computation would not work if sizeof(anything) could be zero.

    More precisely, sizeof a structure type provides the total size in bytes of the structure, including data it contains and any internal or trailing padding (which are used to ensure alignment of data in a manner that is suitable for the target machine). The one byte you are seeing means that your compiler adds one byte padding to an empty struct (the actual amount of padding depends on the implementation).

  4. #4
    Algorithm Dissector iMalc's Avatar
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    Don't worry, it doesn't take up any space when used purely as a base class for another class. Look up "Empty base member optimization".
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  5. #5
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by kollurisrinu View Post
    Hi,
    Can anybody explain why size of a Empty class is 1 byte?
    Every object must have a unique address. So It Is Written In The Standard. But if an object occupies no space at all, how can it have an address? Therefore it must occupy space.

    Also, iMalc's comment about empty base optimization is pertinent. Same goes for empty objects as members of other objects.

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    Quote Originally Posted by brewbuck View Post
    Every object must have a unique address. So It Is Written In The Standard. But if an object occupies no space at all, how can it have an address? Therefore it must occupy space.

    Also, iMalc's comment about empty base optimization is pertinent. Same goes for empty objects as members of other objects.
    Why size of the empty structure is 0 bytes eventhough structure variable also have unique address?

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    I don't recall the standard saying that an empty class/structure (declaration at least) must take up any space. Although, when instantiating it, I suppose it would take a byte... or two.
    (Yes, I know this topic is about 2.5 years old.)
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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