lvalue

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

• Left side of an assignment
• area = height * width
• The lvalue is area
• An lvalue is required as the target of an assignment

rvalue

An expression that is not a lvalue

• Right side of an assignment
• area = height * width
• The rvalue is height * width
• 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 without using pointers
• Previously implemented via swap() type operations
• All containers in the C++ Standard Library include a swap() operation
• If you implement any large object, consider having a swap()

Move Semantics Usage

Implementation Problems

• Need to add a swap() method, and it's difficult to add a method to a class that is not yours
• Need to remember to use the swap() method
• Not actually a move, but a swap where something is going away
• Cannot be used with temporary objects because they are not addressable

Temporary objects are not lvalues

References

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 Benchmark and std::stack::push()