lvalue

An expression that refers to a memory location and therefore we can get the address of via the & operator

• Left side of an assignment
• E.g., int area = height * width, area is the lvalue
• An lvalue is required as the target of an assignment

rvalue

An expression that is not an lvalue

• Right side of an assignment
• E.g., int area = height * width, height * width is the rvalue
• An rvalue cannot be the target of an assignment

Example

• foo() returns an lvalue
• bar() returns an rvalue
• i is an lvalue
• A call to foo() is an lvalue
• Can point to the result of a call to foo()
• Cannot point to the result of a call to bar()

Copy vs. Swap

• Copy field through copy constructor
• Copy field through assignment
• Swap field through swap()

Move Semantics

• Move data instead of copying
• Can efficiently pass data from one part of the program to another, e.g., XMLDocument, without using pointers
• Previously implemented via swap() type operations

Move Semantics Usage

Implementation Problems

• Need to create a swap() method
• Need to remember to use the swap() method
• Not actually a move, but a swap where something is going away
• Cannot use with temporary objects because they are not addressable

Temporary objects are not lvalues

References

• A& - lvalue reference
• A&& - rvalue reference

Current IN Parameters

Current IN Parameters + r-value

swap() Implementation

std::move()

• Converts an lvalue into an rvalue
• Moves the data from the right to the left
• The argument object is left in a "valid but unspecified state"

std::move() Implementation

Using std::move()

Producer Consumer