Getting the data type of an unknown variable

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  1. #1
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    Getting the data type of an unknown variable

    Hi,

    How can I get the data type of an unknown variable?

    For example, I want to do something similar to:

    Code:
    int func(int x)
    {
       if (the data type of x is int)
           return 0;
       else
           return 1;
    }
    How can I implement this function in C?

    I have read somewhere that this cannot be done in C, but I was wondering if there is an equivalent way to check the data type of the argument?

    Thank you.

  2. #2
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    You have to use macros and typeof() for that. Example:
    Code:
    #define func(x) ((typeof(x) == int) ? 0 : 1)
    Not tested, but something like this should work.

  3. #3
    C++ Witch laserlight's Avatar
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    Why do you want to do this?

    In some cases, providing a callback function would be more appropriate.
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  4. #4
    spurious conceit MK27's Avatar
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    Quote Originally Posted by memcpy View Post
    Not tested, but something like this should work.
    It doesn't . Or: typeof() is non-standard, but if you mean the GNU typeof, you can't use it like that. I think it is a macro to start with and what you would end up with is something like "if (int == int)" (or ("if char == int")), which would be an error.

    In any case:

    I have read somewhere that this cannot be done in C
    Right, because it is not logically meaningful:

    Code:
    int func(int x)
    The type of x is declared as int. x is an int, end of story.

    WRT pointers, they may point to data that is not structured the way they have been cast or used, but unless you do it yourself, that data does not contain any type indicator. Ie, for custom types, you can implement your own thing this way (but simply adding a param to the function would be better).

    Personally, I don't see much use value for this, even in languages which do have a real typeof().
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  5. #5
    Algorithm Dissector iMalc's Avatar
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    Such a function does not exist because the type is known and does not need checking. It doesn't exist for the same reason that you don't need to ask yourself each morning "am I human?".

    If you have a void* on the other hand, then that's a different story. Then the answer would be to look at the caller and callee, and design a solution that is sensible.
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  6. #6
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    In C++ you can do something like that with templates (with restrictions), but not in C.

  7. #7
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    Thanks a lot for all these answers. It is clear then that implementing this checking is a waste of time...

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