mardi 24 mai 2016

Is there any use in creating a r-value reference variable using std::move(). [C++]

Can someone help mend my understanding of std::move?

I thought that if an r-value reference goes out of scope what it references would too if it was assigned using the std::move operator. Why is that not the case in the below code?

#include<iostream>
using namespace std;

int main()
{
    string one = "1 - one";
    string two = "2 - two";
    {
        //not as expected
        string & lValRef = one;
        string && rValRef = std::move(two);
        string newS(rValRef);
    }
    cout << "one : " << one << endl;
    cout << "two : " << two << endl;
    {
        //as expected
        string temp(std::move(one));
        string tempAssignment;
        tempAssignment = std::move(two);
    }
    cout << "one : " << one << endl;
    cout << "two : " << two << endl;
    return 0;
}

You can fiddle with it here.

I always thought using std::move was a way to leave objects in a 'deletable state'. So I was surprised that 'two' printed anything out the first time. Is there any use in creating a && r-value reference as I did ('rValRef')? [I understand that a std::move() would be required around my 'rValRef' for it to work as desired].

Below is my own code that I used to help me understand this better. Please play around with it if you desire :) Code here.

#include <iostream>
#include <vector>
using namespace std;

class SimpleClass {
    friend ostream& operator<<(ostream & s,const SimpleClass & rhs);
private:
    vector<char> data;
public:
    SimpleClass(initializer_list<char> lst):data(lst.size()) {
        copy(lst.begin(),lst.end(),data.begin());
    }
    SimpleClass(size_t dim = 0):data(dim){};
    virtual ~SimpleClass() = default;
    SimpleClass(const SimpleClass & rhs) = default;
    SimpleClass & operator=(const SimpleClass & rhs) = default;
    SimpleClass(SimpleClass && rhs):data(move(rhs.data)){};
    SimpleClass & operator=(SimpleClass && rhs){
        if (this != &rhs){
            this->data = move(rhs.data);
            return *this;
        }
    }
};
ostream& operator<<(ostream & s,const SimpleClass & rhs){
    for (size_t i = 0; i != rhs.data.size(); ++i)
        s << rhs.data[i];

    return s;
}
int main()
{
    SimpleClass one = {'o','n','e'};
    SimpleClass two = {'t','w','o'};
    {
        SimpleClass & lValRef = one;
        SimpleClass && rValRef = std::move(two);
    }
    cout << "one : " << one << endl;
    cout << "two : " << two << endl;
    {
        SimpleClass temp(std::move(one));
        SimpleClass tempAssignment;
        tempAssignment = std::move(two);
    }
    cout << "one : " << one << endl;
    cout << "two : " << two << endl;
    return 0;
}

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