avoiding the tedium of optional parameters

If I have a constructor with say 2 required parameters and 4 optional parameters, how can I avoid writing 16 constructors or even the 10 or so constructors I'd have to write if I used default parameters (which I don't like because it's poor self-documentation)? Are there any idioms or methods using templates I can use to make it less tedious? (And easier to maintain?)

Solution 1:

You might be interested in the Named Parameter Idiom.

To summarize, create a class that holds the values you want to pass to your constructor(s). Add a method to set each of those values, and have each method do a return *this; at the end. Have a constructor in your class that takes a const reference to this new class. This can be used like so:

class Person;

class PersonOptions
{
  friend class Person;
  string name_;
  int age_;
  char gender_;

public:
   PersonOptions() :
     age_(0),
     gender_('U')
   {}

   PersonOptions& name(const string& n) { name_ = n; return *this; }
   PersonOptions& age(int a) { age_ = a; return *this; }
   PersonOptions& gender(char g) { gender_ = g; return *this; }
};

class Person
{
  string name_;
  int age_;
  char gender_;

public:
   Person(const PersonOptions& opts) :
     name_(opts.name_),
     age_(opts.age_),
     gender_(opts.gender_)
   {}
};
Person p = PersonOptions().name("George").age(57).gender('M');

Solution 2:

What if you made a parameter object that contained all the fields? Then you could just pass that, and just set whichever fields you need. There's probably a name for that pattern, not sure what it is though...

UPDATE:

Code might look like somewhat this:

paramObj.x=1;
paramObj.y=2;
paramObj.z=3;
paramObj.magic=true;
... //set many other "parameters here"

someObject myObject = new someObject(paramObj);

and inside the someObject constructor you can set defaults for things that were not already set (or raise an error if it was mandatory).

Honestly, I'm not a big fan of this solution, but I've used it once or twice when paramObj made sense by containing a set of data that usually all went together (so we could use it for more than just constructors), and it was better than multiple constructors. I found that it was ugly but it worked, YMMV.

Solution 3:

And now for the "Boost has something for it" answer:

The Boost Parameter Library seems to be a good fit to your use case.

Solution 4:

All new for C++17

#include <optional>

using optional_int = std::optional<int>;

class foo {
    int arg0, arg1; // required
    int arg2, arg3; // optional
    const int default_2 = -2;
    const int default_3 = -3;
public:
    foo(int arg0, int arg1, optional_int opt0 = {}, optional_int opt1 = {})
        : arg0(arg0), arg1(arg1)
        , arg2(opt0.value_or(default_2))
        , arg3(opt1.value_or(default_3))
    { }

};

int main() {
    foo bar(42, 43, {}, 45); // Take default for opt0 (arg2)
    return 0;
}

I have a cubic spline implementation that allows the user optionally to specify the first derivative at either the left end, the right end, or both. If a derivative is not specified, then the code in effect calculates one, by assuming that the second derivative is zero (the so-called "natural spline"). Here is a fragment for the left end.

// Calculate the second derivative at the left end point
    if (!left_deriv.has_value()) {
        ddy[0]=u[0]=0.0; // "Natural spline"
    } else {
        const real yP0 = left_deriv.value();
        ddy[0] = -0.5;
        u[0]=(3.0/(x[1]-x[0]))*((y[1]-y[0])/(x[1]-x[0])-yP0);
    }