Can I write different copyCtor for const and non-const instances?

问题

I have the following problem:

I have a class which should do this:

Obj o;
Obj o1(o), o1=o; // deep-copies
const Obj c(o), c=o; // deep-copies
const Obj c1(c), c1=c; // shallow-copies
Obj o2(c), o2=c; // deep-copies

How can I do this preferably without inheritance? (I mean I would do Const_obj inheriting from Obj otherwise.)

EDIT:

Using o.clone() directly is not an option because then I could easily introduce bugs by accidentally not cloning.

EDIT:

Finally, there is a proper, complete solution with lazy evaluation using the idea from Effective C++ by Scott Meyers. Check out my answer below.

回答1:

No, you can't.

A constructor can not be cv-qualified, so you can't force it to construct a const object.
The return type of a function (including operators) is not a part of it's signature, so you can't overload a function with just changing it's return type.

Also, if it was possible, I would find it really confusing. Just make methods that suit your needs, and name them in an unambiguous way.

回答2:

After reading Effective C++ by Scott Meyers, the following is a solution:

define a template which does a lazy evaluation (with reference counting):

class Obj : private lazy<Obj_data>{};

and the lazy stores the Obj_data privately, has protected accessors, one for modification, one for read-only access.
The modifier accessor first deep-copies the Obj_data if necessary, then hands over the reference to the data. The read-only accessor just returns a const reference.

The overall cost of this is storing 2 extra pointers (one for the data and one for the counter) and a counter.

Implementation is something like this:

class lazy{
protected:
  lazy(const lazy&obj){lazy_copy(obj);}
  //(the required constructors, operator= ...)

  // accessors:
  const Obj_data& data() const {return *od;}
  Obj_data& mod_data() {make_private(); return *od;}
private:
  void lazy_copy(const lazy& obj);
  void make_private(); // this does the actual deep-copy, as late as possible.
private:
  counter*;
  Obj_data* od;
};

So, reading and modifying an attribute of Obj goes

void Obj::method(){
   cout << data().some_attribute;    // simple read
   mod_data().i = 10;                // simple modify
   const Obj_data& const_d = data(); // assignable for lots of read-outs
   Obj_data& var_d = mod_data();     // assignable for lots of modifications.
}

Note that you can only use data() in a const member as mod_data() is a non-const function in the class, so this solution is completely safe with little overhead.

Theory background: the desired behaviour in the question is an implementation detail, does not concern the client. Therefore we solve it by private inheritance.

回答3:

You can, in part, with a dummy argument:

class C {
public:
    struct NonStandardCopy { };

    C (const C &) {
        // "ordinary" copy constructor with default behavior
    }

    C (const C &, NonStandardCopy) {
        // "other" "copy" constructor
    }
};

C c = c1; // default
C c (c1); // default
C c (c1, C::NonStandardCopy ()); // non-default

EDIT: A clone-only approach could be what you want (together with move semantics the performance hit might not be too large):

class C {
private:
    struct DeepCopy { };
    struct ShallowCopy { };

    C (const C &) = delete;

    C (const C &, DeepCopy) {
        // deep copy
    }

    C (const C &, ShallowCopy) {
        // shallow copy
    }
public:
    // move constructor
    C (C && other) = default;

    const C clone () const { // 1
        // shallow copy
        return C (*this, ShallowCopy ());
    }

    C cloneToNonConst () const {  // 2
        // deep copy
        return C (*this, DeepCopy ());
    }

    C clone () { // 3
        return cloneToNonConst ();
    }
};

C o;
C o1 = o.clone (); // call 3
const C o2 = o1.clone (); // call 3
const C o3 = o2.clone (); // call 1
C c4 = o3.cloneToNonConst (); // call 2; o3.clone () will give error

来源：https://stackoverflow.com/questions/13343718/can-i-write-different-copyctor-for-const-and-non-const-instances

标签

c++

copy-constructor

deep-copy

shallow-copy