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Thread: C++ noob question.

  1. #1
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    C++ noob question.

    Hi

    I'm havin' difficulty understanding the difference between the two.
    Code:
    void someFunction(someType obj1);
    and
    Code:
    void sameFunction(someType &obj1)
    I understand that with the first example, when you call
    Code:
    someFunction
    , the parameter is going to be copied to
    Code:
    obj
    . One then needs to use other means which are not
    as expensive.

    Why would one pass the address as opposed to
    passing a pointer --- specificaly, why do copy constructors not take a pointer but a reference instead?

    Is it a matter of preference?
    Last edited by 3v3rgr33n; March 12th, 2013 at 01:56 PM.

  2. #2
    iMac71 is offline Gee! These Aren't Roasted!
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    Re: C++ noob question.

    Perhaps the following two links may help you to understand the difference:
    http://www.cplusplus.com/doc/tutorial/functions/
    http://www.cplusplus.com/doc/tutorial/functions2/

  3. #3
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    Re: C++ noob question.

    Hey

    Thanx for the resources. Let me try to rephrase my question. Why would a person use
    Code:
    void someFunction(someType &var)
    as opposed to
    Code:
    void someFunction(someType *var)
    . Is it a preference thing?

    To my understanding, you cannot manipulate &var directly, you would need to create a pointer (within the scope of someFunction), pointing to that memory address, so why not just pass the pointer in the first place?

  4. #4
    iMac71 is offline Gee! These Aren't Roasted!
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    Re: C++ noob question.

    this additional link may help you to understand the difference of using & instead of *:
    http://www.cplusplus.com/doc/tutorial/pointers/

  5. #5
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    Re: C++ noob question.

    Quote Originally Posted by 3v3rgr33n View Post
    Hey

    Thanx for the resources. Let me try to rephrase my question. Why would a person use
    Code:
    void someFunction(someType &var)
    as opposed to
    Code:
    void someFunction(someType *var)
    . Is it a preference thing?

    To my understanding, you cannot manipulate &var directly, you would need to create a pointer (within the scope of someFunction), pointing to that memory address, so why not just pass the pointer in the first place?
    For the first example the variable is passed by reference. So any changes to the var will be automatically reflected to the original object. In the second example you will need to dereference the pointer in order to manipulate the data it points to.

    Have a look at this page for an explanation for arguments passed by reference vs arguments passed by value.

  6. #6
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    Re: C++ noob question.

    tbh I'm a bit of a noob as well, and learning C as opposed to C++, so I could be wrong (but it sounds like the languages overlap here)...

    I'm fairly sure (that at least in C), *var refers to the value at the address which the pointer, var points to, whereas &var refers to the memory address of var. At one (*), you're passing the value at the memory address of the thing being pointed to, and with the other (&), you're passing the memory address of the pointer (which points to the memory address of whatever it is var points to) into the function.

    Hope that helps

  7. #7
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    Re: C++ noob question.

    @usernamer: C++ and C do have the same syntax for working with pointers, but this is an additional feature of C++. It's a prefix to a function parameter or variable that says it is a reference. It uses the same "&" character as the "address of" in C, but it's not the same thing.

    @OP: In the case of passing what you might call an "out" parameter, obviously the syntax of the pointer and reference is slightly different:

    Pointer
    Code:
    void increment(int *x)
    {
            *x = *x + 1;
    }
    
    int main()
    {
            int value = 0;
            increment(&value);
            cout << value << endl;
    }
    Reference
    Code:
    void increment(int &x)
    {
            x = x + 1;
    }
    
    int main()
    {
            int value = 0;
            increment(value);
            cout << value << endl;
    }
    The result is the same in both cases.

    But there's more to it than syntax. A reference is like a pointer with conditions attached:

    • It can't be NULL
    • It can't be reassigned to refer to something else
    • It can't be manipulated, e.g. incremented to point at the next entry in an array

    Sometimes it's more appropriate to work with a pointer, e.g. if you want to work with the idea of null, or if you want to manipulate it. Other times it's a better fit to work with a reference. In the increment function above, I think a reference fits the intent of the function better than a pointer. (I don't want to manipulate it and it shouldn't be null).
    Please create new threads for new questions.
    Please wrap code in code tags using the '#' button or enter it in your post like this: [code]...[/code].

  8. #8
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    Re: C++ noob question.

    Quote Originally Posted by usernamer View Post
    I'm fairly sure (that at least in C), *var refers to the value at the address which the pointer, var points to, whereas &var refers to the memory address of var. At one (*), you're passing the value at the memory address of the thing being pointed to, and with the other (&), you're passing the memory address of the pointer (which points to the memory address of whatever it is var points to) into the function.
    No you have it wrong. You should read more about pointers, but more specifically about declaring functions that take arguments and how you declare(and read) the type of each argument. If you omit the variable name this is more obvious. So for example

    Code:
    void myfunction(int);
    Takes one argument of type int.

    Code:
    void myfunction(int*);
    Takes one argument which is pointer to int
    Code:
    void myfunction(int&);
    Take one argument of type int but instead of copying the value it references it (think pointer magic is happening behind the scenes)

    And to show the different usages of &
    Code:
    int x = 10;
    int *y = NULL; //pointer to int
    y = &x; //y now points to same value of x
    
    void myfunction(int &z)
    {
       z = 15;
    }
    
    myfunction(x); //notice that you passed an int to the function and not an pointer to int.
    //after this function call x will have a value of 15 and not 10
    Also & can be used as a bitwise operator but this will confuse the hell out of you, so I don't provide an example.

    Keep in mind that in C++ (and probably in C) the same keyword can have a different meaning depending on the context.

    EDIT: r-senior beat me to it.

  9. #9

    Re: C++ noob question.

    tl;dr - Don't learn C and C++ at the same time.

  10. #10
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    Re: C++ noob question.

    Just don't learn C++ ...

    <Turns laptop off and scurries off to bed>
    Please create new threads for new questions.
    Please wrap code in code tags using the '#' button or enter it in your post like this: [code]...[/code].

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