passing object by reference in C

Question

The usual way to pass a variable by reference in C   also C  is as follows   void  someFunction dataType  name      dataType e g int char float etc        definition       int main        dataType v       somefunction  amp v      address of variable v being passed     return 0      But to my surprise  I noticed when passing an object by reference the name of object itself serves the purpose no  amp  symbol required  and that during declaration definition of function no   symbol is required before the argument  The following example should make it clear      this  include  lt iostream gt  using namespace std   class CDummy     public      int isitme  CDummy amp  param         why not  CDummy  param       int CDummy  isitme  CDummy amp  param      if   amp param    this  return true    else return false     int main        CDummy a    CDummy  b    amp a    if   b- gt isitme a                    why not isitme  amp a      cout  lt  lt   yes   amp a is b     return 0      I have problem understanding why is this special treatment done with class   Even structures which are almost like a class are not used this way  Is object name treated as address as in case of arrays

User · Answer

One thing that I have to add is that there is no reference in C.

Secondly, this is the language syntax convention. & - is an address operator but it also mean a reference - all depends on usa case

If there was some "reference" keyword instead of & you could write

int CDummy::isitme (reference CDummy param)

but this is C++ and we should accept it advantages and disadvantages...

User · Answer

Passing by reference in the above case is just an alias for the actual object    You ll be referring to the actual object just with a different name   There are many advantages which references offer compared to pointer references

User · Answer

Ok  well it seems that you are confusing pass-by-reference with pass-by-value  Also  C and C   are different languages  C doesn t support pass-by-reference   Here are two C   examples of pass by value      ex 1 int add int a  int b        return a   b        ex 2 void add int a  int b  int  result         result   a   b     void main         int result   0          ex 1     result   add 2 2      result will be 4 after call         ex 2     add 2 3  amp result      result will be 5 after call     When ex 1 is called  the constants 2 and 2 are passed into the function by making local copies of them on the stack  When the function returns  the stack is popped off and anything passed to the function on the stack is effectively gone   The same thing happens in ex 2  except this time  a pointer to an int variable is also passed on the stack  The function uses this pointer  which is simply a memory address  to dereference and change the value at that memory address in order to  return  the result  Since the function needs a memory address as a parameter  then we must supply it with one  which we do by using the  amp   address-of  operator on the variable result   Here are two C   examples of pass-by-reference      ex 3 int add int  amp a  int  amp b        return a b        ex 4 void add int  amp a  int  amp b  int  amp result        result   a   b     void main         int result   0          ex 3     result   add 2 2      result   2 after call        ex 4     add 2 3 result      result   5 after call     Both of these functions have the same end result as the first two examples  but the difference is in how they are called  and how the compiler handles them   First  lets clear up how pass-by-reference works  In pass-by-reference  generally the compiler implementation will use a  pointer  variable in the final executable in order to access the referenced variable   or so seems to be the consensus  but this doesn t have to be true  Technically  the compiler can simply substitute the referenced variable s memory address directly  and I suspect this to be more true than generally believed  So  when using a reference  it could actually produce a more efficient executable  even if only slightly   Next  obviously the way a function is called when using pass-by-reference is no different than pass-by-value  and the effect is that you have direct access to the original variables within the function  This has the result of encapsulation by hiding the implementation details from the caller  The downside is that you cannot change the passed in parameters without also changing the original variables outside of the function  In functions where you want the performance improvement from not having to copy large objects  but you don t want to modify the original object  then prefix the reference parameters with const   Lastly  you cannot change a reference after it has been made  unlike a pointer variable  and they must be initialized upon creation   Hope I covered everything  and that it was all understandable

User · Answer

A reference is really a pointer with enough sugar to make it taste nice        But it also uses a different syntax to pointers  which makes it a bit easier to use references than pointers  Because of this  we don t need  amp  when calling the function that takes the pointer - the compiler deals with that for you  And you don t need   to get the content of a reference    To call a reference an alias is a pretty accurate description - it is  another name for the same thing   So when a is passed as a reference  we re really passing a  not a copy of a - it is done  internally  by passing the address of a  but you don t need to worry about how that works  unless you are writing your own compiler  but then there are lots of other fun things you need to know when writing your own compiler  that you don t need to worry about when you are just programming     Note that references work the same way for int or a class type

User · Answer

What seems to be confusing you is the fact that functions that are declared to be pass-by-reference  using the  amp   aren t called using actual addresses  i e   amp a    The simple answer is that declaring a function as pass-by-reference   void foo int amp  x     is all we need  It s then passed by reference automatically   You now call this function like so   int y   5  foo y     and y will be passed by reference   You could also do it like this  but why would you  The mantra is  Use references when possible  pointers when needed      include  lt iostream gt  using namespace std   class CDummy   public      int isitme  CDummy  param        int CDummy  isitme  CDummy  param        if  param    this  return true      else return false     int main          CDummy a      CDummy  b    amp a                 assigning address of a to b     if   b- gt isitme  amp a              Called with  amp a  address of a  instead of a         cout  lt  lt   yes   amp a is b       return 0      Output   yes   amp a is b

[c++] passing object by reference in C++

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