Class variables: public access read-only, but priv

问题:

Whoopee, not working on that socket library for the moment. I'm trying to educate myself a little more in C++.

With classes, is there a way to make a variable read-only to the public, but read+write when accessed privately? e.g. something like this:

class myClass {
    private:
    int x; // this could be any type, hypothetically

    public:
    void f() {
        x = 10; // this is OK
    }
}

int main() {
    myClass temp;

    // I want this, but with private: it's not allowed
    cout << temp.x << endl;


    // this is what I want:

    // this to be allowed
    temp.f(); // this sets x...

    // this to be allowed
    int myint = temp.x;

    // this NOT to be allowed
    temp.x = myint;
}

My question, condensed, is how to allow full access to x from within f() but read-only access from anywhere else, i.e. int newint = temp.x; allowed, but temp.x = 5; not allowed? like a const variable, but writable from f()...

EDIT: I forgot to mention that I plan to be returning a large vector instance, using a getX() function would only make a copy of that and it isn't really optimal. I could return a pointer to it, but that's bad practice iirc.

P.S.: Where would I post if I just want to basically show my knowledge of pointers and ask if it's complete or not? Thanks!

回答1:

Of course you can:

class MyClass
{
    int x_;

public:
    int x() const { return x_; }
};

If you don't want to make a copy (for integers, there is no overhead), do the following:

class MyClass
{
    std::vector<double> x_;

public:
    const std::vector<double>& x() const { return x_; }
};

This does not make any copy. It returns a reference to const.

回答2:

While I think a getter function that returns const T& is the better solution, you can have almost precisely the syntax you asked for:

class myClass {
    private:
    int x_; // Note: different name than public, read-only interface

    public:
    void f() {
        x_ = 10; // Note use of private var
    }
    const int& x;
    myClass() : x_(42), x(x_) {} // must have constructor to initialize reference
};

int main() {
    myClass temp;

    // temp.x is const, so ...
    cout << temp.x << endl; // works
    // temp.x = 57;  // fails

}

EDIT: With a proxy class, you can get precisely the syntax you asked for:

class myClass {
public:

    template <class T>
    class proxy {
        friend class myClass;
    private:
        T data;
        T operator=(const T& arg) { data = arg; return data; }
    public:
        operator const T&() const { return data; }
    };

    proxy<int> x;
    // proxy<std::vector<double> > y;


    public:
    void f() {
        x = 10; // Note use of private var
    }
};

temp.x appears to be a read-write int in the class, but a read-only int in main.

回答3:

A simple solution, like Rob, but without contructor:

class myClass {
    private:
    int m_x=10; // Note: different name than public, read-only interface
    public:
    const int& x=m_x;

};

int main() {
    myClass temp;

    // temp.x is const, so ...
    cout << temp.x << endl; // works
    // temp.x = 57;  // fails

}

Is like a get methode, but shorter. Interesant question ... something like. extent const bool member; can save a lot of getters ...but I don't know languages with this feature...

回答4:

This may do what you want.

If you want a readonly variable but don't want the client to have to change the way they access it, try this templated class:

template<typename MemberOfWhichClass, typename primative>                                       
class ReadOnly {
    friend MemberOfWhichClass;
public:
    inline operator primative() const                 { return x; }

    template<typename number> inline bool   operator==(const number& y) const { return x == y; } 
    template<typename number> inline number operator+ (const number& y) const { return x + y; } 
    template<typename number> inline number operator- (const number& y) const { return x - y; } 
    template<typename number> inline number operator* (const number& y) const { return x * y; }  
    template<typename number> inline number operator/ (const number& y) const { return x / y; } 
    template<typename number> inline number operator<<(const number& y) const { return x <<y; }
    template<typename number> inline number operator>>(const number& y) const { return x >> y; }
    template<typename number> inline number operator^ (const number& y) const { return x ^ y; }
    template<typename number> inline number operator| (const number& y) const { return x | y; }
    template<typename number> inline number operator& (const number& y) const { return x & y; }
    template<typename number> inline number operator&&(const number& y) const { return x &&y; }
    template<typename number> inline number operator||(const number& y) const { return x ||y; }
    template<typename number> inline number operator~() const                 { return ~x; }

protected:
    template<typename number> inline number operator= (const number& y) { return x = y; }       
    template<typename number> inline number operator+=(const number& y) { return x += y; }      
    template<typename number> inline number operator-=(const number& y) { return x -= y; }      
    template<typename number> inline number operator*=(const number& y) { return x *= y; }      
    template<typename number> inline number operator/=(const number& y) { return x /= y; }      
    template<typename number> inline number operator&=(const number& y) { return x &= y; }
    template<typename number> inline number operator|=(const number& y) { return x |= y; }
    primative x;                                                                                
};      

Example Use:

class Foo {
public:
    ReadOnly<Foo, int> x;
};

Now you can access Foo.x, but you can't change Foo.x! Remember you'll need to add bitwise and unary operators as well! This is just an example to get you started

回答5:

There is a way to do it with a member variable, but it is probably not the advisable way of doing it.

Have a private member that is writable, and a const reference public member variable that aliases a member of its own class.

class Foo
{
  private:
      Bar private_bar;

  public:
      const Bar& readonly_bar; // must appear after private_bar
                              // in the class definition

  Foo() :
       readonly_bar( private_bar )
  {
  }
};

That will give you what you want.

void Foo::someNonConstmethod()
{
    private_bar.modifyTo( value );
}

void freeMethod()
{
    readonly_bar.getSomeAttribute();
}

What you can do, and what you should do are different matters. I'm not sure the method I just outlined is popular and would pass many code reviews. It also unnecessarily increases sizeof(Foo) (albeit by a small amount) whereas a simple accessor "getter" would not, and can be inlined, so it won't generate more code either.

回答6:

You may want to mimic C# properties for access (depending what you're going for, intended environment, etc.).

class Foo
{
  private:
    int bar;

  public:
    __declspec( property( get = Getter ) ) int Bar;

    void Getter() const
    {
      return bar;
    }
}

回答7:

You would have to leave it private and then make a function to access the value;

private:

    int x;

public:

    int X()
    {
        return x;
    }

回答8:

You need to make the member private and provide a public getter method.

回答9:

The only way I know of granting read-only access to private data members in a c++ class is to have a public function. In your case, it will like:

int getx() const { return x; }

or

int x() const { return x; }.

By making a data member private you are by default making it invisible (a.k.a no access) to the scope outside of the class. In essence, the members of the class have read/write access to the private data member (assuming you are not specifying it to be const). friends of the class get access to the private data members.

Refer here and/or any good C++ book on access specifiers.

回答10:

Write a public getter function.

int getX(){ return x; }

回答11:

but temp.x = 5; not allowed?

This is any how not allowed in the snippet posted because it is anyhow declared as private and can be accessed in the class scope only.

Here are asking for accessing

cout << temp.x << endl;

but here not for-

int myint = temp.x;

This sounds very contradictory.