classes, structs and unions

This is a discussion on classes, structs and unions within the C++ Programming forums, part of the General Programming Boards category; Hey I was wondering what is the difference between classes unions and structs? They all seem to be the same...

  1. #1
    Imperator of Darkness Luciferek's Avatar
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    classes, structs and unions

    Hey
    I was wondering what is the difference between classes unions and structs?
    They all seem to be the same

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    Classes and structs are identical except a struct's members are visible by default (public) and a class's is private by default.
    However, many C++ programmers tend to only use structs for POD, plain old data and not as classes, objects.
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    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

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    Kernel hacker
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    And a union is different from both struct and class in that ALL of the data members in a union are placed at the same postion [so they are overlaying each other in the same place in memory].

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    Mats
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    Registered User valaris's Avatar
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    Quote Originally Posted by matsp View Post
    And a union is different from both struct and class in that ALL of the data members in a union are placed at the same postion [so they are overlaying each other in the same place in memory].

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    Dumb question - How would that work? How would adding to one variable not erase another if they are all overlayed?

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    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by valaris View Post
    Dumb question - How would that work? How would adding to one variable not erase another if they are all overlayed?
    Writing one variable does erase all the others. The whole point of a union is that you can have a, or b, or c, but only one at a time.

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    Quote Originally Posted by valaris View Post
    Dumb question - How would that work? How would adding to one variable not erase another if they are all overlayed?
    It only works for POD variables -- those without destructors. It doen't do anything to "erase" the variable when written over; it just writes over it. After such a write, only the new variable may be accessed.
    It is too clear and so it is hard to see.
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  7. #7
    Ethernal Noob
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    wonder who named it union then. A preacher perhaps, surely not a mathematician.
    Here to Deceive, Inveigle, Obfuscate Since 1945

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    Quote Originally Posted by indigo0086 View Post
    wonder who named it union then. A preacher perhaps, surely not a mathematician.
    What name would you have chosen?

  9. #9
    Ethernal Noob
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    touche.

    I don't know, "rug" perhaps?
    Here to Deceive, Inveigle, Obfuscate Since 1945

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    If you draw a Venn diagram of the variables in your memory, it is a union.

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    Registered User valaris's Avatar
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    Ahhh...so of all the data types in the union, the union can only be one of these?

  12. #12
    and the hat of sweating
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    A union such as:
    Code:
    union abc
    {
       int i;
       char c;
       float f;
    };
    simply causes i, c & f to begin at the same memory address.

    If you set i to 0x01020304, then read c, you should see 0x01 or 0x04 (depending on whether you're on a Big Endian or Little Endian system).
    Just think of it as having a block of memory and just casting it to int, char, or float...

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    Quote Originally Posted by valaris View Post
    Ahhh...so of all the data types in the union, the union can only be one of these?
    Yes.

    For example,
    Code:
    union  MyUnion  { int n; double d; };
    struct MyStruct { int n; double d; };
    The union allows you to store an integer OR a double (but not both at the same time). The size of the union should be 8 bytes, the first 4 are shared. That means writing to one variable will 'overwrite' the other variable.

    The struct allows you to store an integer AND a double at the same time, because it makes enough room for both variables, which should be 12 bytes.

  14. #14
    Registered User valaris's Avatar
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    Ahh I understand now...
    I don't see them much, what are they used for generally?
    What are they useful for?

  15. #15
    The larch
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    I've seen them used in Windows API. For example the INPUT_RECORD struct looks like this:

    Code:
    typedef struct _INPUT_RECORD { // ir 
    
        WORD EventType; 
        union { 
            KEY_EVENT_RECORD KeyEvent; 
            MOUSE_EVENT_RECORD MouseEvent; 
            WINDOW_BUFFER_SIZE_RECORD WindowBufferSizeEvent; 
            MENU_EVENT_RECORD MenuEvent; 
            FOCUS_EVENT_RECORD FocusEvent; 
        } Event; 
    } INPUT_RECORD;
    This single struct can store data about various events, each requiring different fields. Which event record contains valid data is determined by EventType, other records are just garbage.

    In C++ and high level programming unions are probably very rarely used if ever.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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