Acessing a struct after passing to a function

This is a discussion on Acessing a struct after passing to a function within the C++ Programming forums, part of the General Programming Boards category; Hello to everyone again. how to I access my struct variables after i have passed it to a funtion? I ...

  1. #1
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    Acessing a struct after passing to a function

    Hello to everyone again.
    how to I access my struct variables after i have passed it to a funtion?

    I made this struct.
    Code:
    struct structy
    {
    	long * one;
    	long   two;
    	long   Three;
    };
    
    using namespace std;
    
    void func1 (structy* structy);
    
    int main()
    {
    }
    
    void func1 (structy* structy)
    	{
    
    	cout<<" the var is " <<structy.one<<endl;//trying to access one here
    	
    	
    	}

    This is the error I get.
    Code:
    	
    structhelp.cpp: In function `void func1(structy*)':
    structhelp.cpp:29: error: request for member `one' in `structy', which is of non-class type `structy*'
    What am I doing wrong?

  2. #2
    Software Developer jverkoey's Avatar
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    structy is a pointer, therefor you need to use the -> operater when accessing members, instead of the . operator.

    structy->one instead of structy.one

  3. #3
    Frequently Quite Prolix dwks's Avatar
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    The -> operator is a shortcut for dereferencing (*) and member access (.) combined. Thus, structure->member is the same as (*structure).member.

    You could avoid using pointers (in C++ at least) by using a reference.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
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  4. #4
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    how to I change the value of what the pointer is pointing to?
    In my program I want to use the one variable as a sort of counter. when i try to increment it I am just messing up the the pointer.(I think)

    Code:
    struct structy
    {
    	long * one;
    	long   two;
    	long   Three;
    };
    
    using namespace std;
    
    void func1 (structy* structy);
    
    int main()
    {
    }
    
    void func1 (structy* structy)
    	{
                      structy->one++//problem here.  how to I ++ the value
                                              //not the address?
    	}

  5. #5
    Software Developer jverkoey's Avatar
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    In that code given, you're increment one which is a pointer. Assuming you want to increment the value this is pointing to you have to dereference the pointer as follows:

    (*structy->one)++;

    Using the magic wand of dereferencing as binky would put it.

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    The reason for this is that ++ has a higher precedence than *. = and += are lower, so if you don't like the parenthesis, you can use
    Code:
    *structy->one += 1;
    or
    Code:
    *structy->one = *structy->one + 1;
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


    Other boards: DaniWeb, TPS
    Unofficial Wiki FAQ: cpwiki.sf.net

    My website: http://dwks.theprogrammingsite.com/
    Projects: codeform, xuni, atlantis, nort, etc.

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