CODEWITHSUNDEEP
c++move-semanticsrvalue-reference
Michael
Michael

Reputation: 7809

Get an rvalue when calling a getter method on an rvalue object

Suppose, I have the following code. There's a copy constructor in B which calls a method which copies the resources of a.

Now I also have a move constructor. In this case, a should not be copied but just "steal" the resources from an existing a. Therefore, I also implemented an init taking an rvalue. But of course, when I try to call it with parameter b.a, this is an lvalue...

Is there a way to call this method?

class A{

    A(const A&& a){
        // 'steal' resources from a
    }

    void init(A& a){
       // init this A from another A by copying its resources
    }

    void init(A&& a){
      // init this A from another A stealing its resources and tell the other a, it must not destroy resources upon destruction
    }
};

class B{
    A a;

    B(B& b){
      a.init(b.a)          
    }

    B(B&& b){
      a.init(b.a); // How to call init(A&& a)?  
    }

};

Upvotes: 2

Views: 329

Answers (1)

juanchopanza
juanchopanza

Reputation: 227390

b.a is an lvalue, so you need to apply std::move:

a.init(std::move(b.a));

Note: But why is b an lvalue in the body of B(B&& b)?

Here, the parameter type B&& b simply means that this constructor overload will be chosen over, say, B(const B& b) when invoked with an rvalue. 

B make_B() { return B(); }
B b1(make_B());            // B(B&&) chosen
B b2(b);                   // B(const B&) chosen

But the parameter itself is an lvalue because it has a name. All std::move does is make its argument look like an rvalue.

Upvotes: 7

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