CODEWITHSUNDEEP
c++reference
Xu Hui
Xu Hui

Reputation: 1265

Is there any way can help code return value is const reference?

Suppose I have a class A, the class is copyable, but it will take a lot to copy it.

Suppose I have a class B, it can return a const reference of A.

class B
{
public:
...
    const A& obtainA() const
    {
       return a;
    }

private:
    A a;
};

When I want to use A from B, I think the best practice is accessed A in B by const reference

B b;
const A& a = b.obtainA(); // do not needs to the heavy copy operation of A, good.

However, I always forget the const reference, my code looks like follow.

B b;
A a = b.obtainA(); // leads to the heavy copy operation of A, not good

It not only leads to the heavy copy operation but also the unsafety. If the a call the non-const member function by mistake, it will leads to the member a in b be changed out of class.

Is there exists any way can help me to return const reference? I want something like, if not returned by reference, the code can not be compiled. Or any other advice is OK.

Thanks for your time.

Concrete applications

A in my application is the shared array. The data stored in the array is shared, it just something like the std::shared_ptr, but manage array of object but not single object.

The array is designed can be shared to reduce the copy overhead. The A is copyable because the array should be copyable.

The array is a widely used class, thus it is often a member of Array's user, that is the B in this question.

Upvotes: 1

Views: 65

Answers (2)

cigien
cigien

Reputation: 60208

You could make A non-copyable, and provide a method to do the copy explicitly.

class A {
  private:
    A(A const &);  // expensive copy
  public:
    A make_copy() { return *this; }
};

The make_copy method will create the expensive copy, but other code will no be able to make copies implicitly, since the copy-constructor is private.

Upvotes: 2

Christopher Yeleighton
Christopher Yeleighton

Reputation: 454

If you do not want an object to be modified, make the object itself constant:

B const b;
A a = b.obtainA(); // nope, not allowed

I personally believe that everything should be constant unless there is a good reason for it not to be.

Upvotes: 1

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