How do I remove code duplication between similar const and non-const member functions?

Question

Let\'s say I have the following class X where I want to return access to an internal member:

class Z
{
    // details
};

class X
{
    std::vec         


        

Answer 1

Didn't find what I was looking for, so I rolled a couple of my own...

This one is a little wordy, but has the advantage of handling many overloaded methods of the same name (and return type) all at once:

struct C {
  int x[10];

  int const* getp() const { return x; }
  int const* getp(int i) const { return &x[i]; }
  int const* getp(int* p) const { return &x[*p]; }

  int const& getr() const { return x[0]; }
  int const& getr(int i) const { return x[i]; }
  int const& getr(int* p) const { return x[*p]; }

  template<typename... Ts>
  auto* getp(Ts... args) {
    auto const* p = this;
    return const_cast<int*>(p->getp(args...));
  }

  template<typename... Ts>
  auto& getr(Ts... args) {
    auto const* p = this;
    return const_cast<int&>(p->getr(args...));
  }
};

If you have only one const method per name, but still plenty of methods to duplicate, then you might prefer this:

  template<typename T, typename... Ts>
  auto* pwrap(T const* (C::*f)(Ts...) const, Ts... args) {
    return const_cast<T*>((this->*f)(args...));
  }

  int* getp_i(int i) { return pwrap(&C::getp_i, i); }
  int* getp_p(int* p) { return pwrap(&C::getp_p, p); }

Unfortunately this breaks down as soon as you start overloading the name (the function pointer argument's argument list seems to be unresolved at that point, so it can't find a match for the function argument). Although you can template your way out of that, too:

  template<typename... Ts>
  auto* getp(Ts... args) { return pwrap<int, Ts...>(&C::getp, args...); }

But reference arguments to the const method fail to match against the apparently by-value arguments to the template and it breaks. Not sure why.Here's why.

Answer 2

For a detailed explanation, please see the heading "Avoid Duplication in const and Non-const Member Function," on p. 23, in Item 3 "Use const whenever possible," in Effective C++, 3d ed by Scott Meyers, ISBN-13: 9780321334879.

Here's Meyers' solution (simplified):

struct C {
  const char & get() const {
    return c;
  }
  char & get() {
    return const_cast<char &>(static_cast<const C &>(*this).get());
  }
  char c;
};

The two casts and function call may be ugly but it's correct. Meyers has a thorough explanation why.

Answer 3

I'd suggest a private helper static function template, like this:

class X
{
    std::vector<Z> vecZ;

    // ReturnType is explicitly 'Z&' or 'const Z&'
    // ThisType is deduced to be 'X' or 'const X'
    template <typename ReturnType, typename ThisType>
    static ReturnType Z_impl(ThisType& self, size_t index)
    {
        // massive amounts of code for validating index
        ReturnType ret = self.vecZ[index];
        // even more code for determining, blah, blah...
        return ret;
    }

public:
    Z& Z(size_t index)
    {
        return Z_impl<Z&>(*this, index);
    }
    const Z& Z(size_t index) const
    {
        return Z_impl<const Z&>(*this, index);
    }
};

Answer 4

Typically, the member functions for which you need const and non-const versions are getters and setters. Most of the time they are one-liners so code duplication is not an issue.

Answer 5

How about moving the logic into a private method, and only doing the "get the reference and return" stuff inside the getters? Actually, I would be fairly confused about the static and const casts inside a simple getter function, and I'd consider that ugly except for extremely rare circumstances!

Answer 6

To add to the solution jwfearn and kevin provided, here's the corresponding solution when the function returns shared_ptr:

struct C {
  shared_ptr<const char> get() const {
    return c;
  }
  shared_ptr<char> get() {
    return const_pointer_cast<char>(static_cast<const C &>(*this).get());
  }
  shared_ptr<char> c;
};