如何使对象深度只读

Question

How can I make a class, which is sometimes readonly, and sometimes writable? One option is with getter/setters and a flag which indicates if the object is read-only, but that is a lot of overhead. I also desire this readonly property to work deeply on the object, making sure that all of the objects it contains are also readonly or writeable. Here's some example code of the desired behavior which I tried but failed to achieve using const.

This question is quite generic, so has probably been asked before, but I wasn't able to find a good solution to this exact problem on stackoverflow.

Example code: https://ideone.com/4cXyNF

class InnerClass {
 public:
  InnerClass(int j) : j_(j) {}
  int j_;
};

class OuterClass {
 public:
  OuterClass(int i, InnerClass& c) : i_(i), reference_(c), pointer_(&c) {}
  int i_;
  InnerClass& reference_;
  InnerClass* pointer_;
};

int main() {
    InnerClass c(1);

    OuterClass readwrite(2, c);
    // Desire these 3 operations to work on the writable object
    readwrite.i_ = 3;
    readwrite.reference_.j_ = 4;
    readwrite.pointer_->j_ = 5;

    const OuterClass readonly(6, c);
    // COMPILER ERROR: error: assignment of member 'OuterClass::i_'
    // in read-only object
    // readonly.i_ = 7;
    // Desire this to be a compiler error, but it isn't
    readonly.reference_.j_ = 8;
    // Desire this to be a compiler error, but it isn't
    readonly.pointer_->j_ = 9;

    return 0;
}

Answer 1

If you change your members to functions, you can create const overloads of the methods like this

class InnerClass {
public:

  explicit
  InnerClass(int j) : j_(j) {}

  int& j() { return j_; }
  const int& j() const { return j_; } 
private:
  int j_;
};

class OuterClass {
public:

  OuterClass(int i, InnerClass& c) : i_(i), reference_(c), pointer_(&c) {}

  int& i() { return i_; }
  const int& i() const { return i_; } 

  InnerClass const& reference() const { return reference_; };
  InnerClass & reference()  { return reference_; };

  InnerClass const* pointer() const { return pointer_; };
  InnerClass * pointer() { return pointer_; };

private:
  int i_;

  InnerClass& reference_;
  InnerClass* pointer_;
};

int main() {
    InnerClass c(1);

    OuterClass readwrite(2, c);
    // Desire these 3 operations to work on the writable object
    readwrite.i() = 3;
    readwrite.reference().j() = 4;
    readwrite.pointer()->j() = 5;

    const OuterClass readonly(6, c);
    // COMPILER ERROR: error: assignment of member 'OuterClass::i_'
    // in read-only object
    readonly.i_ = 7;
    // Desire this to be a compiler error, and it is
    readonly.reference().j() = 8;
    // Desire this to be a compiler error, and it is
    readonly.pointer()->j() = 9;

    return 0;
}

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Answer 2

You can achieve this by using member functions to return the references / pointers const if the object itself is const .

class InnerClass {
 public:
  InnerClass(int j) : j_(j) {}
  int j_;
};

class OuterClass
{
  InnerClass& reference_;
public:
  OuterClass(int i, InnerClass& c) : i_(i), reference_(c) {}
  int i_;
  InnerClass & in() { return reference_; }
  InnerClass const & in() const { return reference_; }
};

Now neither i_ nor in().j_ is writable in case outer is const :

InnerClass i{ 1 };
OuterClass write(2, i);
write.i_ = 3; // works
write.in().j_ = 3; // works
OuterClass const read(2, i);
read.i_ = 3; // error!
read.in().j_ = 3; // error!

Answer 3

This is a similar solution as some else already posted, but uses a slightly different approach:

class InnerClass {
 public:
  InnerClass(int j) : j_(j) {}
  int j_;
};

template<bool readOnly>
class OuterClass{
 public:
  OuterClass(int i, InnerClass& c) : i_(i), reference_(c), pointer_(&c) {}
  int i_;
  typename std::conditional<readOnly,const InnerClass&, InnerClass&>::type reference_;
  typename std::conditional<readOnly,const InnerClass* const, InnerClass*>::type pointer_;
};

int main(int argc,char** args){
    InnerClass c(1);
    OuterClass<true> readonly(12,c);
    //readonly.reference_.j_ = 1;   //Error "reference_ is read only"
    //readonly.pointer_->j_ = 1;    //Error "pointer_ is read only"
    OuterClass<false> write(12,c);
    write.reference_.j_ = 1;
    write.pointer_->j_ = 1;
}