mercredi 2 septembre 2015

Move and Forward cases use

I followed this tutorial to start to understand the move semantics and rvalue references in C++11. At some point, he implements these two classes with the std::move in the move constructors explaining that

we pass the temporary to a move constructor, and it takes on new life in the new scope. In the context where the rvalue expression was evaluated, the temporary object really is over and done with. But in our constructor, the object has a name; it will be alive for the entire duration of our function. In other words, we might use the variable other more than once in the function, and the temporary object has a defined location that truly persists for the entire function. It's an lvalue in the true sense of the term locator value

class MetaData
{
public:
    MetaData(int size, const string& name)
        : _name(name)
        , _size(size)
    {}

    MetaData(const MetaData& other)
        : _name(other._name)
        , _size(other._size)
    {
        cout << "MetaData -- Copy Constructor" << endl;
    }

    MetaData(MetaData&& other)
        : _name(move(other._name))
        , _size(other._size)
    {
        cout << "MetaData -- Move Constructor" << endl;
    }

  ~MetaData()
  {
    _name.clear();
  }

    string getName() const { return _name; }
    int getSize() const { return _size; }

private:
    string _name;
    int _size;
};

class ArrayWrapper
{
public:
    ArrayWrapper()
        : _p_vals(new int[64])
        , _metadata(64, "ArrayWrapper")
    {}

    ArrayWrapper(int n)
        : _p_vals(new int[n])
        , _metadata(n, "ArrayWrapper")
    {}

    ArrayWrapper(ArrayWrapper&& other)
        : _p_vals(other._p_vals)
        , _metadata(move(other._metadata))
    {
        cout << "ArrayWrapper -- Move Constructor" << endl;
        other._p_vals = nullptr;
    }

    ArrayWrapper(const ArrayWrapper& other)
        : _p_vals(new int[other._metadata.getSize()])
        , _metadata(other._metadata)
    {
        cout << "ArrayWrapper -- Copy Constructor" << endl;
        for (int i = 0; i < _metadata.getSize(); ++i)
            _p_vals[i] = other._p_vals[i];
    }

    ~ArrayWrapper()
    {
        delete[] _p_vals;
    }

    int* getVals() const { return _p_vals; }
    MetaData getMeta() const { return _metadata; }

private:
    int* _p_vals;
    MetaData _metadata;
};

In the ArrayWrapper move constructor I tried to change std::move with std::forward<MetaData> and the code shows that if I call the ArrayWrapper move constructor this will call the MetaData move constructor, like the example with the std::move.

Of course if I don't use either std::move or std::forward the MetaData copy costructor will be called.

The question is, in this case, is there a difference between using std::move and std::forward? Why should I use one instead of the other?

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