How to pass a struct by reference and by value, please?

Question

3.00/5 (1 vote)

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for example I have this one;

struct students {
   string name;
   int ID;
   char Grade;
};

students undergraduates,graduates;

I want to pass it to a function what to do? thanks :)

Posted 1-May-11 10:18am

console_programing

Updated 1-Mar-22 18:31pm

Sergey Alexandrovich Kryukov

v3

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Albert Holguin 1-May-11 16:36pm

edit: added pre tags

2 solutions

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-05-01T11:19:00

Solution 2

Here is how:

C++

struct Student { //use singular, not plural
   //...
}

Student first, second; 

void PassByValue(Student who); //prototype
void PassByReference(Student& who); //prototype

PassByValue(first); //you work with a shallow copy of "who, if you changes is, actual object remains the same
PassByReference(second); //no copy (faster), when you modify who you modify actual passed object

Call is identical, but second call always needs a variable.
Structure or not — no matter, can be any type.

—SA

Posted 1-May-11 11:19am

Sergey Alexandrovich Kryukov

Updated 3-May-11 19:37pm

v2

Comments

Albert Holguin 1-May-11 17:24pm

good answer

Sergey Alexandrovich Kryukov 1-May-11 17:25pm

Thank you, Albert.
--SA

Albert Holguin 1-May-11 17:26pm

I answered that one hastily :)

Sergey Alexandrovich Kryukov 1-May-11 17:46pm

:-)

Legor 3-May-11 7:16am

Shouldn't your call to PassByReference() look like PassByReference(&first)?

Sergey Alexandrovich Kryukov 4-May-11 1:35am

Thank you for the note; I just copied the line from previous line of code, will fix my mistake.
Thanks again.
--SA

Legor 4-May-11 2:56am

Your welcome, but actually i think it's not right the way written in your post so far.

If you declare
Student first;

The call to the reference function should look like:
PassByReference(&first);

Otherwise if you declare first as a pointer:
Student* first;

You could call the function like:
PassByReference(first);

Sergey Alexandrovich Kryukov 4-May-11 3:08am

No. This time it's written correctly. &first in your sample is not reference, it is an address; those are different thing. There are not pointers in these sample. A sample with the pointer would be another one, when you pass a ___pointer by value___:

void PassByPointerByValue(Student* who);
Student student;
PassByPointerByValue(&student);

--SA

Legor 4-May-11 3:17am

Yes you're right, i somehow mixed this up with C pass by reference syntax.

Sergey Alexandrovich Kryukov 4-May-11 15:24pm

OK, right, this is explainable. When I got your comment I was confused myself a bit as by that time I already forgot what language it was. I think now it's all clear.
Thank you.
--SA

mbue · Accepted Answer · 2011-05-02T04:43:00

C#

typedef struct _STUDENT
{
   string    name;
   int      ID;
   char      Grade;
} STUDENT;
void fnStruct(STUDENT student)
{
  // you can everything you want
  // the value from caller is not affected
}
void fnStructRef(STUDENT& student)
{
  // you can read or write values
  // the value from the caller can be modified
  // usually a ref cannot be a null pointer
}
void fnStructConstRef(const STUDENT& student)
{
  // you can only read the members
  STUDENT*  writable = (STUDENT*)&student;
  // this construct makes it possible to full access
  // the value from the caller will be modified
}
void fnStructPtr(STUDENT* student)
{
  // you can read or write values
  // the value from the caller can be modified
  // the pointer can be a null pointer
  if(student)
  {
    // the param is not null
  }
}
void fnStructConstPtr(const STUDENT* student)
{
  // you can only read the members
  // the pointer can be a null pointer
  if(student)
  {
    // the param is not null
    STUDENT*  writable = (STUDENT*)student;
    // this construct makes it possible to full access
    // the value from the caller will be modified
  }
}
void main()
{
  STUDENT   undergraduates,
            graduates;
  // parameter cannot be changed in any circumstances
  fnStruct(undergraduates);
  // parameter can be changed
  fnStructRef(undergraduates);
  // parameter should not be changed, but it can
  fnStructConstRef(undergraduates);
  // parameter can be changed
  fnStructPtr(&undergraduates);
  // parameter should not be changed, but it can
  fnStructConstPtr(&undergraduates);
}

Regards.