Default value to a parameter while passing by reference in C

Question

Is it possible to give a default value to a parameter of a function while we are passing the parameter by reference  in C    For example  when I try to declare a function like   virtual const ULONG Write ULONG  amp State   0  bool sequence   true     When I do this it gives an error      error C2440   default argument    cannot convert from  const int  to  unsigned long  amp     A reference that is not to  const  cannot be bound to a non-lvalue

User · Answer

There are two reasons to pass an argument by reference: (1) for performance (in which case you want to pass by const reference) and (2) because you need the ability to change the value of the argument inside the function.

I highly doubt that passing an unsigned long on modern architectures is slowing you down too much. So I'm assuming that you're intending to change the value of State inside the method. The compiler is complaining because the constant 0 cannot be changed, as it's an rvalue ("non-lvalue" in the error message) and unchangeable (const in the error message).

Simply put, you want a method that can change the argument passed, but by default you want to pass an argument that can't change.

To put it another way, non-const references have to refer to actual variables. The default value in the function signature (0) is not a real variable. You're running into the same problem as:

struct Foo {
    virtual ULONG Write(ULONG& State, bool sequence = true);
};

Foo f;
ULONG s = 5;
f.Write(s); // perfectly OK, because s is a real variable
f.Write(0); // compiler error, 0 is not a real variable
            // if the value of 0 were changed in the function,
            // I would have no way to refer to the new value

If you don't actually intend to change State inside the method you can simply change it to a const ULONG&. But you're not going to get a big performance benefit from that, so I would recommend changing it to a non-reference ULONG. I notice that you are already returning a ULONG, and I have a sneaky suspicion that its value is the value of State after any needed modifications. In which case I would simply declare the method as so:

// returns value of State
virtual ULONG Write(ULONG State = 0, bool sequence = true);

Of course, I'm not quite sure what you're writing or to where. But that's another question for another time.

User · Answer

This little template will help you   template lt typename T gt  class ByRef   public      ByRef                ByRef const T value    mValue value               operator T amp    const           return  T amp  mValue          private      T mValue       Then you ll be able to   virtual const ULONG Write ULONG  amp State   ByRef lt ULONG gt  0   bool sequence   true

User · Answer

There still is the old C way of providing optional arguments  a pointer that can be NULL when not present   void write  int  optional   0         if  optional   optional   5

User · Answer

Define 2 overload functions  virtual const ULONG Write ULONG  amp State  bool sequence   true    virtual const ULONG Write bool sequence   true        int State   0      return Write State  sequence

User · Answer

It s possible with const qualifier for State   virtual const ULONG Write const ULONG  amp State   0  bool sequence   true

User · Answer

No  it s not possible   Passing by reference implies that the function might change the value of the parameter  If the parameter is not provided by the caller and comes from the default constant  what is the function supposed to change

User · Answer

void f const double amp  v     double   NULL      if   amp v    NULL      cout  lt  lt   default   lt  lt  endl    else     cout  lt  lt   other    lt  lt  v  lt  lt  endl

User · Answer

You can do it for a const reference  but not for a non-const one  This is because C   does not allow a temporary  the default value in this case  to be bound to non-const reference   One way round this would be to use an actual instance as the default   static int AVAL   1   void f  int  amp  x   AVAL           stuff     int main          f             equivalent to f AVAL       but this is of very limited practical use

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In case of OO    To say that a Given Class has and  Default  means that this Default  value  must declared acondingly an then may be usd as an Default Parameter ex   class Pagination   public      int currentPage                Pagination             currentPage   1                             your Default Pagination     static Pagination amp  Default             static Pagination pag          return pag             On your Method       shared ptr lt vector lt Auditoria gt   gt    findByFilter Auditoria amp  audit  Pagination amp  pagination   Pagination  Default         This solutions is quite suitable since in this case   Global default Pagination  is a single  reference  value  You will also have the power to change default values at runtime like an  gobal-level  configuration ex  user pagination navigation preferences and etc

User · Answer

You cannot use a constant literal for a default parameter for the same reason you cannot use one as a parameter to the function call   Reference values must have an address  constant references values need not  ie they can be r-values or constant literals    int  foo  int amp  i        return  amp i     foo 0      compiler error   const int  bar   const int amp  i        return  amp i     bar 0      ok    Ensure that you re default value has an address and you re fine   int null object   0   int Write int  amp state   null object  bool sequence   true       if   amp state     amp null object                 called with default paramter       return sequence  1  rand            else               called with user parameter       state    sequence  1  rand          return state           I ve used this pattern a few times where I had a parameter that could be a variable or null   The regular approach is to have the user pass in a pointer this is case   They pass in a NULL pointer if they don t want you to fill in the value   I like to null object approach   It makes the callers life easier without terribly complicating the callee code

User · Answer

Another way could be the following   virtual const ULONG Write ULONG  amp State  bool sequence   true       wrapper const ULONG Write bool sequence   true       ULONG dummy     return Write dummy  sequence       then the following calls are possible   ULONG State  object- gt Write State  false      sequence is false   returns  State object- gt Write State      assumes sequence   true   returns  State object- gt Write false      sequence is false  no  return  object- gt Write       assumes sequence   true  no  return

User · Answer

I think not  and the reason is that default values are evaluated to constants and values passed by reference must be able to change  unless you also declare it to be constant reference

User · Answer

virtual const ULONG Write ULONG  amp State   0  bool sequence   true     The answer is quite simple and I am not so good on explaining but if you want to pass a default value to a non-const parameter which probably will be modified in this function is to use it like this   virtual const ULONG Write ULONG  amp State     ULONG  0  bool sequence    gt  true

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void revealSelection const ScrollAlignment amp    ScrollAlignment  alignCenterIfNeeded  bool revealExtent   false

User · Answer

I have a workaround for this  see the following example on default value for int amp    class Helper   public      int x      operator int amp      return x           How to use it  void foo int  amp x   Helper           You can do it for any trivial data type you want  such as bool  double

User · Answer

There is also rather dirty trick for this   virtual const ULONG Write ULONG  amp  amp State   0  bool sequence   true     In this case you have to call it with std  move   ULONG val   0  Write std  move val      It is only some funny workaround  I totally do not recommend it using in real code

User · Answer

It has been said in one of the direct comments to your answer already  but just to state it officially   What you want to use is an overload   virtual const ULONG Write ULONG  amp State  bool sequence   inline const ULONG Write       ULONG state    bool sequence   true    Write  state  sequence       Using function overloads also have additional benefits   Firstly you can default any argument you wish   class A      class B      class C      void foo  A const  amp   B const  amp   C const  amp    void foo  B const  amp   C const  amp       A defaulted void foo  A const  amp   C const  amp       B defaulted void foo  C const  amp       A  amp  B defaulted etc      It is also possible to redefine default arguments to virtual functions in derived class  which overloading avoids   class Base   public    virtual void f1  int i   0       default  0     virtual void f2  int     inline void f2          f2 0                           equivalent to default of  0          class Derived   public Base  public    virtual void f1  int i   10       default  10     using Base  f2    virtual void f2  int       void bar        Derived d    Base  amp  b  d     d f1           10  used   b f1           0  used    d f2          f1 int  called with  0     b f2          f1 int  called with  0     There is only one situation where a default really needs to be used  and that is on a constructor   It is not possible to call one constructor from another  and so this technique does not work in that case

[c++] Default value to a parameter while passing by reference in C++

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