【C++ 补习】Copy Control

C++ Primer 5th edition, chapter 13.

The Rule of Three

• If a class needs a destructor, it almost surely needs a copy constructor and copy-assignment operator as well.

• If a class needs a copy constructor, it almost surely needs a copy-assignment operator, and vice versa.

The Copy and Swap Idiom

Classes that define swap often use swap to define their assignment operator. These operators use a technique known as copy and swap. This technique swaps the left-hand operand with a copy of the right-hand operand. The interesting thing about this technique is that it automatically handles self assignment and is automatically exception safe.

Rvalue References

An rvalue reference is a reference that must be bound to an rvalue. Rvalue references have the important property that they may be bound only to an object that is about to be destroyed. As a result, we are free to "move" resources from an rvalue reference to another object.

As we know, we cannot bind rvalue references to expressions that require a conversion, to literals, or to expressions that return an rvalue. Rvalue references have the opposite binding properties: We can bind an rvalue reference to these kinds of expressions, but we cannot directly bind an rvalue reference to an lvalue.

The Synthesized Move Operations

If a class defines its own copy constructor, copy-assignment operator, or destructor, the move constructor and move assignment operator are not synthesized. As a result, some classes do not have a move constructor or a move-assignment operator.

The compiler will synthesize a move constructor or a move-assignment operator only if the class doesn't define any of its own copy-control members and if every nonstatic data member of the class can be moved. The compiler can move members of built-in type. It can also move members of a class type if the members's class has the corresponding move operation.