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In C#, passing a parameter by reference can't be ignored:

// C#
void foo(ref int p_value) ; // passing an int by reference

void bar()
{
   int i = 0 ;
   foo(ref i) ;
}

In C++ (and as far as I am concerned, it is a good thing), the passing by reference is only marked in the function prototype, so the code above becomes:

// C++
void foo(int & p_value) ; // passing an int by reference

void bar()
{
   int i = 0 ;
   foo(i) ;     // at call site, there is no "ref" marker.
}

The thing is, for a few special functions, to ease code reviewing, we would want to use a C#-like solution to read at call site if a variable is expected to be modified or not.

I devised a solution wrapping the value with a struct, something like:

template<typename T>
struct Ref
{
   T & m_value ;
   explicit Ref(T & p_value) : m_value(p_value) {}

   operator T & () { return this->m_value ; }
   operator const T & () const { return this->m_value ; }
} ;

template <typename T>   // wrapping T with Ref
Ref<T> byRef(T & p_value)
{
   return Ref<T>(p_value) ;
}

template <typename T>   // idempotent-enabling overload (no need for Ref<Ref<T>>)
Ref<T> & byRef(Ref<T> & p_value)
{
   return p_value ;
}

I expect this trivial code to be inlined away in optimized code.

This enables me to write the code:

void foo(Ref<int> p_int)
{
   p_int *= 10 ;
}

void bar()
{
   int i = 0 ;
   foo(byRef(i)) ;
}

Which is exactly what I want:

  • no use of foo(& i) C-style notation, with pointers, needing to handle NULL, etc.
  • in the cases we care, we can see in the code what is passed by non-const reference

Did I miss some unexpected problem/hidden cost that will come back to bite me in a few weeks/months/years?

Could this code be written better?

Will the code be confusing when seen through some un*x debuggers?

P.S.: Of course, Ref and byRef will be properly namespaced. I'm still not sure about making Ref::m_value public or private.

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1 Answer

I can't see anything I would do majorly different.

  • I would make the member T & m_value private.
  • I would change the name of the method byRef() to just ref()
    • Note you still have a difference in identifiers.
      The class is Ref while the function is ref()

The only problem I see is that at inside the function, the object is not automatically usable in its native type you need to cast it back before you can use it:

struct X { void print();};

void print(Ref<X> x)
{
    x.print();  // This fails to compile

    // To use the x object I need to cast it back to an X
    static_cast<X&>(x).print();

    // If we are going to use it multiple times then we could set up
    // a local reference to the object as this will generate an implicit cast
    X&  x1 = x;
    x1.print();
}

Thus getting at methods in the object becomes slightly on the messy side.

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I get your point about the method calls. I didn't think about that. . . o.O . . . The good thing is that the aim is to wrap built-ins. . . And of course, you're right about the names: byRef should become ref. –  paercebal Nov 17 '12 at 17:41
    
Note that keeping m_value public (and giving it a short name) would enable direct manipulation of the variable, including the method calls. As the Ref is used only as a simple wrapper, there's no concern about encapsulation, there. Am I wrong? –  paercebal Nov 17 '12 at 17:45
    
@paercebal: It was only a minor thing (personally I would make it private). BUT I have no strong argument for my perspective; in a code review at work if another engineer really wanted it public I would be fine with that as well. Direct manipulation (via member or casting) boils down to the same thing. There is some extra messing around you need to do. I am sure we can take this one step further (if we use some template/macro hijinks). –  Loki Astari Nov 17 '12 at 18:36
    
If the design goal is only to support built-in types then you should make that explicit by makeing sure it does not compile for other types. –  Loki Astari Nov 17 '12 at 18:39
    
makeing sure it does not compile for other types : You're right. On my computer, my own proof-of-concept had additional basic constraints, but I'm unsure about the exact level of conformance/efficiency of all the compilers at my new job (started since two weeks ago... ^_^...), so I'm not sure I'll apply that "patch" until I'm more familiar about those (I already broke the build once for a dumb warning, not sure I want to try it again soon)... ^_^ –  paercebal Nov 18 '12 at 12:05
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