Pass by value/reference

This is a discussion on Pass by value/reference within the C Programming forums, part of the General Programming Boards category; Hi In the following example, is the variable integer being passed by reference or by value? Code: void test(int* var) ...

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    Pass by value/reference

    Hi

    In the following example, is the variable integer being passed by reference or by value?

    Code:
    void test(int* var) {
        *var++;
    }
    
    int main() {
        int integer = 1;
        test(&integer);
        printf("%d", integer);
        return 0;
    }
    The reason why I ask is because according to Pass By Value - Definition, pass by reference is not possible in C (since there are no references).

    Best,
    Niles.

  2. #2
    C++ Witch laserlight's Avatar
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    The variable named integer is not passed at all. Rather, the address of the variable named integer is passed by value. This is used to simulate passing the variable named integer by reference.
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    Quote Originally Posted by Niels_M View Post
    according to Pass By Value - Definition, pass by reference is not possible in C
    This is true - all function arguments are passed by value. However, the variable integer is not what's being passed - what's being passed is the address of the variable "integer". This address is indeed being passed by value, so you could say it's "simulating" pass-by-reference.

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    Hi. I dont understand why.
    according to Pass By Value - Definition, pass by reference is not possible in C
    I thought variables could be either passed by Value/Reference?

    Reference - Taking reference from original variable's address
    Value - Taking concurrent value of particular variable

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    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Inneart
    I thought variables could be either passed by Value/Reference?

    Reference - Taking reference from original variable's address
    Value - Taking concurrent value of particular variable
    Yes, and then there are other parameter passing schemes that are possible. However, that does not mean that all those parameter passing schemes are directly available in every programming language.
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    C++まいる!Cをこわせ! Elysia's Avatar
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    It's really misnamed. It's not pass-by-reference. It's pass-by-address. The address of the variable is copied into the function.
    Pass-by-value copies the variable value into the function.
    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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    So in C language, variables are always passed by duplicating its value?

    C = 5
    Pass C to a F(x)
    C of F(x) duplicates value of C

    And if I want it to modify the original C,

    C = 5
    Pass &C to a F(x)
    C of F(x) chops address of original C

    Is that right?

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Chops off?
    The last example passes the address of C to F(x), allowing F(x) to write to the address instead of the local C to reflect the changes to the caller.
    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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    Chop as in take.

    So my examples are correcto?

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    C++まいる!Cをこわせ! Elysia's Avatar
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    In that case, yes.
    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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    Quote Originally Posted by laserlight View Post
    The variable named integer is not passed at all. Rather, the address of the variable named integer is passed by value. This is used to simulate passing the variable named integer by reference.
    So pass by reference is just the adress being passed by value? So it is OK so say that C has pass by reference as well?

  12. #12
    C++まいる!Cをこわせ! Elysia's Avatar
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    Pass by reference is a concept and not a language feature. So in a sense, it is OK to do that.
    I think it's silly, though. Clearly it isn't passing by reference, since the address is passed.
    Furthermore, C++ also has a pass-by-reference language feature.

    Conclusion: whether it is acceptable or not is up to you.
    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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    Quote Originally Posted by Elysia View Post
    Pass by reference is a concept and not a language feature. So in a sense, it is OK to do that.
    I think it's silly, though. Clearly it isn't passing by reference, since the address is passed.
    Furthermore, C++ also has a pass-by-reference language feature.

    Conclusion: whether it is acceptable or not is up to you.
    So what happens when we in C++ have pass-by-reference (I know how to pass a variable by reference, but what actually happens)? Does the address get copied or what?

  14. #14
    C++まいる!Cをこわせ! Elysia's Avatar
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    It's pass-by-reference. The new variable in the function is an alias of the one you passed in. That is what the standard says.
    In practice, it is usually implemented using pointers. But again, it doesn't have to be.
    Thus, it is really a "true" pass-by-reference feature.
    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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    Quote Originally Posted by Niels_M View Post
    So pass by reference is just the adress being passed by value? So it is OK so say that C has pass by reference as well?
    Basically that's all that pass by reference is in any language... you're giving the address of the variable instead of the value of the variable. C is just more honest about it.

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