COMS W4995 C++ Deep Dive for C Programmers

Move Semantics

Lecture outline

1. defining and calling createIntArray()

   • does not compile because we deleted the copy constructor

   • explain that the copy constructor would have to deep-copy

   • returning a large object by value is a very nice and clean pradigm (think Java), but old C++ avoided it due to the cost of copying

2. move constructor

   • motivation: make returning local objects efficient

   • if you are copying from an object that will be destroyed, just steal the internals instead of copying from it

   • code: copy the interal pointer to the new object, and then sever the connection from the old object

   • enabling copy elision eliminates the move constructor calls

3. rvalue reference

   • ravlue examples: 5, MyString{"abc"}

   • rvalue-test.cpp: regular ref vs. const ref vs. rvalue ref

   • why does move constructor need rvalue reference?

     • regular reference cannot be bound to rvalue
     • const reference can be, but you cannot then change the object

4. move assignment

   • code: self-assign check, delete mine, copy rhs, sever rhs, return *this
   • casting to rvalue reference
   • std::move()

5. Essential 6, Rule of Five, and Rule of Zero