 

Category: functors   Component type: concept 
Description
A Monoid Operation is a special sort of BinaryFunction. A BinaryFunction must satisfy three conditions in order to be a Monoid Operation. First, its first argument type and second argument type must be the same, and its result type must be the same as its argument type. Second, there must be an identity element. Third, the operation must be associative. Examples of Monoid Operations are addition and multiplication. [1]
Refinement of
BinaryFunction
Associated types
Argument type  The type of the Monoid Operation's first argument and second argument, and also the type returned when the Monoid Operation is returned. 
Notation
F  A type that is a model of MonoidOperation 
T  F 's argument type. 
f  Object of type F 
x , y , z  Objects of type T 
Definitions
A type F
that is a model of BinaryFunction is associative if F
's first argument type, second argument type, and result type are the same, and if, for every object f
of type F
and for every objects x
, y
, and z
of F
's argument type, f(x, f(y, z))
is the same as f(f(x, y), z)
. [2]
Valid Expressions
In addition to the expressions described in the BinaryFunction requirements, the following expressions must be valid.
Name  Expression  Type requirements  Return type 
Function call  f(x, y)   T 
Identity element  identity_element(f) [3]   T 
Expression semantics
Name  Expression  Precondition  Semantics  Postcondition 
Function call  f(x, y)  x and y are in the domain of f .  Calls f with x and y as arguments.  
Identity element  identity_element(f)   Returns the monoid's identity element. That is, the return value is a value id of type T such that, for all x in the domain of f , f(x, id) and f(id, x) both return x .  
Complexity guarantees
Invariants
Associativity  For any x , y , and z of type T , f(x, f(y, z)) and f(f(x, y), z) return the same value. [4] 
Identity element.  There exists some element id of type T such that, for all x of type T , f(x, id) and f(id, x) both return x . The expression identity_element(f) returns id . 
Models
Notes
[1] A monoid is one of three closely related algebraic structures. A semigroup is a set S, and a binary operation *, with the properties that * is closed on S (that is, if x and y are elements of S then x * y is also a member of S) and that * is associative (that is, if x, y, and z are elements of S, then x * (y * z) = (x * y) * z). A monoid is a semigroup that has an identity element. That is, there exists some element id such that, for all x in S, x * id = id * x = x. Finally, a group is a monoid with the property that every element has an inverse. That is, for every x in S, there exists an element xi such that x * xi = xi * x = id. As an example, the set of real numbers under multiplication is a monoid (the identity element is 1), but it isn't a group. It isn't a group because 0 has no inverse.
[2] Mathematics textbooks typically write this as an equation, instead of using words like "is the same as". We can't use equality in this definition, however, because F
's argument type might not be EqualityComparable. If F
's argument type is EqualityComparable, however, then these two expression are expected to be equal: the condition of associativity becomes f(x, f(y, z)) == f(f(x, y), z)
[3] This is implemented as an overloaded function. The function identity_element
is defined, in the standard header functional, and the nonstandard backwardcompatibility header function.h, for arguments of type plus<T>
and multiplies<T>
. If you define a new Monoid Operation F
(matrix multiplication, for example), you must overload identity_element
for arguments of type F
. The identity_element
function is an SGI extension; it is not part of the C++ standard.
[4] Associativity is not the same as commutativity. That is, the requirement that x * (y * z) == (x * y) * z
is completely unrelated to the requirement that x * y == y * x
. Monoid operations are required to be associative, but they are not required to be commutative. As an example, square matrices under multiplication form a monoid even though matrix multiplication is not commutative.
See also
BinaryFunction, plus
, multiplies