Thread: move semantics / rvalue references

just now looking into this. not sure if i'm doign it wrong. a sanity check would be much appreciated.

Code:

Matrix&& Matrix::operator +(const Matrix& b)const
{
    Matrix result(*this);
    result+=b;
    return (Matrix&&)result;
}

Matrix::Matrix(Matrix&& rhs):
   data(rhs.data),
   rows(rhs.rows),
   cols(rhs.cols)
{
}

void foo()
{
   Matrix a;
   Matrix b;
   Matrix aPlusB(a+b);
}

edit: ok it seems like i am definitely missing the point. this is the usage case i'd like to accomodate: using arithmetic operators on matrix operations without any extraneous copies. if someone can assist in disabusing me of my ignorance that would be fantastic.

thanks.

Code:

Matrix::Matrix(Matrix&& rhs):
   data(std::move(rhs.data)),
   rows(std::move(rhs.rows)),
   cols(std::move(rhs.cols))
{
}

Matrix aPlusB(a+b); //aPlus is moved from the temp object.

es, it is "dangling" reference that return a reference to a local object.
The copy constructor of Matrix would not be invoked, because you give a move constructor. But the move constructor you defined is not really "move sematics", becuase its members are copy constructed, so you have to use std::move() to transform the members of rhs into rvalue.

yes, but it is not necessary using std::move() with bar, because bar is a POD type.

In fact, move constrictor is not a substitute for copy constrcutor. Don't forget copy constrictor if there is a deep copy.

You return locals by value, and if there is a move constructor, this will be invoked to move the result to the caller.

A move constructor is a constructor that takes a reference to a temporary object; nothing more, nothing less.
Previously we could only use const references to temporary objects.
Using the new move constructor, the goal is to "steal" the resources from the temp object. Don't forget to properly "remove" the resources from the temporary (like settings pointers to nullptr) to ensure it doesn't destroy them.

Remember it's still a reference, and you can't return a reference to a local variable!