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 &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 &' A reference that is not to 'const' cannot be bound to a non-lvalue

Answer 1

In my usecase I needed to create an empty context to be passed as reference. Especially when I use a constructor I an not create it before like so:

void foo() {
   ctx_t ctx;
   foo(ctx);
}
void foo(ctx_t&);

So the solution is to create one function that accepts ctx_t and default initializes it. And pass this object down as reference.

Foo::Foo(ctx_t ctx = {}) : Foo(ctx) {}
Foo::Foo(ctx_t& ctx) {}

Answer 2

Upgrading our code base from C++17 to C++20 using MSVC, Microsoft now finally enforces this rule.

The semantics for void foo(int& optOutParam = 0) would be optOutParam is an optional output parameter. Often I see this concrete example, where multiple output parameters are required:

bool Initialise(Window& wnd, Context& ctxt, std::string& error = "") {
// ...
}

The return value indicates whether the function succeeded, while error contains the reason why it might have failed. Unfortunately std::optional is explicitly forbidden for references, or this would be a great solution, too.

std::optional<int&> opt; // compiler error

A good way to refactor is to actually have the function return all values and refrain from using reference parameters as output.

std::tuple<Window, Context, std::string> Initialise();

void modernCpp() {
  auto[wnd, ctxt, errorMsg] = Initialise();
  if(!errorMsg.empty()) {
    exit(1);
  }
}

void olderCpp() {
  Window wnd;
  Context ctxt;
  std::string errorMsg;
  std::tie(wnd, ctxt, errorMsg) = Initialise();

  // or
  tuple<Window, Context, std::string> result = Initialise();
}

Unfortunately this means having to refactor every call-site as well, which might be a huge job in large code bases. To deal with that case I use an r-value overload, that discards the error message. No call-site has to be changed then.

bool Initialise(Window& wnd, Context& ctxt, std::string& error) {
  // default value removed
}
bool Initialise(Window& wnd, Context& ctxt, std::string&& discardedErrorMsg = "") {
  // call overload above
  return Initialise(wnd, ctxt, discardedErrorMsg);
}

This has one major draw-back, namely gathering the info for error message might be very costly. Asking a database what went wrong, for example, may require another network trip. Bear in mind though, that the original function has the same issue, so this is a great micro-optimisation chance.

For the vast majority of new functions with multiple out parameters, I use multiple return values.

Answer 3

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 &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 &, B const &, C const &);
void foo (B const &, C const &); // A defaulted
void foo (A const &, C const &); // B defaulted
void foo (C const &); // A & 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 & 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.

Answer 4

Define 2 overload functions.

virtual const ULONG Write(ULONG &State, bool sequence = true);

virtual const ULONG Write(bool sequence = true)
{
    int State = 0;
    return Write(State, sequence);
}

Answer 5

I have a workaround for this, see the following example on default value for int&:

class Helper
{
public:
    int x;
    operator int&() { return x; }
};

// How to use it:
void foo(int &x = Helper())
{

}

You can do it for any trivial data type you want, such as bool, double ...

Answer 6

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;
}

Answer 7

There is also rather dirty trick for this:

virtual const ULONG Write(ULONG &&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!

Answer 8

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.

Answer 9

It's possible with const qualifier for State:

virtual const ULONG Write(const ULONG &State = 0, bool sequence = true);

Answer 10

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& Default() {
        static Pagination pag;
        return pag;
    }
};

On your Method ...

 shared_ptr<vector<Auditoria> > 
 findByFilter(Auditoria& audit, Pagination& 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..

Answer 11

void f(const double& v = *(double*) NULL)
{
  if (&v == NULL)
    cout << "default" << endl;
  else
    cout << "other " << v << endl;
}

Answer 12

void revealSelection(const ScrollAlignment& = ScrollAlignment::alignCenterIfNeeded, bool revealExtent = false);

Answer 13

virtual const ULONG Write(ULONG &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 &State = *(ULONG*)0, bool sequence =
> true);

Answer 14

Another way could be the following:

virtual const ULONG Write(ULONG &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->Write(State, false); // sequence is false, "returns" State
object->Write(State); // assumes sequence = true, "returns" State
object->Write(false); // sequence is false, no "return"
object->Write(); // assumes sequence = true, no "return"

Answer 15

This little template will help you:

template<typename T> class ByRef {
public:
    ByRef() {
    }

    ByRef(const T value) : mValue(value) {
    }

    operator T&() const {
        return((T&)mValue);
    }

private:
    T mValue;
};

Then you'll be able to:

virtual const ULONG Write(ULONG &State = ByRef<ULONG>(0), bool sequence = true);

Answer 16

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& i )
{
   return &i;
}

foo(0); // compiler error.

const int* bar ( const int& i )
{
   return &i;
}

bar(0); // ok.

Ensure that you're default value has an address and you're fine.

int null_object = 0;

int Write(int &state = null_object, bool sequence = true)
{
   if( &state == &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.

Answer 17

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 & x = AVAL ) {
   // stuff
} 

int main() {
     f();       // equivalent to f(AVAL);
}

but this is of very limited practical use.

Answer 18

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?

Answer 19

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.