initializing of strucutre variable

This is a discussion on initializing of strucutre variable within the C Programming forums, part of the General Programming Boards category; hi, why cant we initialise structure variables at the time structure declaration. Code: struct { int i=10; char a='s'; };...

  1. #1
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    initializing of strucutre variable

    hi,

    why cant we initialise structure variables at the time structure declaration.
    Code:
    struct
    {
         int i=10;
         char a='s';
    };

  2. #2
    Deathray Engineer MacGyver's Avatar
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    Because that's the way the language was designed.

  3. #3
    Algorithm Dissector iMalc's Avatar
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    Exactly what I was going to say.
    If you want to do that kind of thing, you want to be using C++, in which you can use "constructors" to achieve the desired effect.
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  4. #4
    Deathray Engineer MacGyver's Avatar
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    And constructors can be "faked" in C.

    Code:
    struct something
    {
        int x;
    };
    
    ...
    
    struct something * create_something(int x)
    {
        struct something *s = malloc(sizeof(*s));
        s->x = x;
        return s;
    }
    So.... yeah....

  5. #5
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    Code:
    struct
    {
         int i;
         char a;
    } instance = { 10, 's' };
    
    int main()
    {
        printf ("%d %c\n", instance.i, instance.a);
        return 0;
    }

  6. #6
    Deathray Engineer MacGyver's Avatar
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    Yes, on a per instance basic. Not default values, though, like the OP appears to want.

  7. #7
    C++まいる!Cをこわせ!
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    Think of it this way:
    Structures are blueprints on how to build your custom data type. The thing is that it's a blueprint - it doesn't contain information about what the actual objects created from it contains. So you can't modify the blueprint to hold information.
    Instead, you must first CREATE your object FROM the blueprint and initialize it, which is what robwhit just did.

  8. #8
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    thank you for info.i was getting doubt on it. now iam able to understand.

  9. #9
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by MacGyver View Post
    And constructors can be "faked" in C.
    That's more like a combination of new plus a constructor call. I'm not sure I'd call it "fake," it's an extremely common pattern.

    I tend to decouple allocation from initialization, and deallocation from deinitialization, for maximum flexibility.

  10. #10
    Deathray Engineer MacGyver's Avatar
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    You are right that is new + constructor. I put fake in quotation marks because I didn't think it was the right word, but it still fit the idea. It's not a constructor, but it can be thought of as one.

    Also, I believe C++ really does use these kind of techniques, though, under the hood more or less. So the word fake becomes less appropriate if that is the case.

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