The Orderable
concept represents totally ordered data types.
Intuitively, Orderable
objects must define a binary predicate named less
returning whether the first argument is to be considered less than the second argument. The word "total" means that distinct objects must always be ordered; if a
and b
are not equal, then exactly one of less(a, b)
and less(b, a)
must be true. This is a contrast with weaker kinds of orders that would allow some objects to be incomparable (neither less than nor greater than). Also note that a nonstrict total order may always be obtained from a strict total order (and viceversa) by setting
The nonstrict version is used in the description of the laws because it makes them easier to parse for humans, but they could be formulated equivalently using the strict order.
less
When less
is defined, the other methods are defined from it using the same definition as mandated in the laws below.
Rigorously speaking, a total order <=
on a set S
is a binary predicate \( <= \;: S \times S \to bool \) such that for all a
, b
, c
in S
,
Additionally, the less
, greater
and greater_equal
methods should have the following intuitive meanings:
Comparable
(free model)Orderable
requires less_equal
to be a total order, a model of Comparable
may always be obtained by setting hana::integral_constant
, hana::optional
, hana::pair
, hana::string
, hana::tuple
Two data types T
and U
that model the crosstype version of the usual LessThanComparable C++ concept are automatically a model of Orderable
by setting
The crosstype version of the LessThanComparable concept is analogous to the crosstype version of the EqualityComparable concept presented in N3351, which is compatible with the usual single type definition. However, note that the LessThanComparable concept only requires <
to be a strict weak ordering, which is a weaker requirement than being a total order. Hence, if less
is used with objects of a LessThanComparable data type that do not define a total order, some algorithms may have an unexpected behavior. It is the author's opinion that defining operator<
as a nontotal order is a bad idea, but this is debatable and so the design choice of providing a model for LessThanComparable data types is open to debate. Waiting for some user input.
Let A
and B
be two Orderable
data types. A function \( f : A \to B\) is said to be orderpreserving (also called monotone) if it preserves the structure of the Orderable
concept, which can be rigorously stated as follows. For all objects x
, y
of data type A
,
Another important property is that of being orderreflecting, which can be stated as
We say that a function is an orderembedding if it is both orderpreserving and orderreflecting, i.e. if
The comparison methods (less
, less_equal
, greater
and greater_equal
) are "overloaded" to handle distinct data types with certain properties. Specifically, they are defined for distinct data types A
and B
such that
A
and B
share a common data type C
, as determined by the common
metafunctionA
, B
and C
are all Orderable
when taken individuallyis_embedding
metafunction.The method definitions for data types satisfying the above properties are
The less
, greater
, less_equal
and greater_equal
methods can be called in two different ways. First, they can be called like normal functions:
However, they may also be partially applied to an argument as follows:
Take good note that the order of the arguments is reversed, so for example less.than(x)(y)
is equivalent to less(y, x)
, not less(x, y)
. This is because those variants are meant to be used with higher order algorithms, where the chosen application order makes sense.
Variables  
constexpr auto  boost::hana::greater 
Returns a Logical representing whether x is greater than y . More...  
constexpr auto  boost::hana::greater_equal 
Returns a Logical representing whether x is greater than or equal to y . More...  
constexpr auto  boost::hana::less 
Returns a Logical representing whether x is less than y . More...  
constexpr auto  boost::hana::less_equal 
Returns a Logical representing whether x is less than or equal to y . More...  
constexpr auto  boost::hana::max 
Returns the greatest of its arguments according to the less ordering. More...  
constexpr auto  boost::hana::min 
Returns the smallest of its arguments according to the less ordering. More...  
constexpr auto  boost::hana::ordering 
Returns a function performing less after applying a transformation to both arguments. More...  

constexpr 
#include <boost/hana/fwd/greater.hpp>
Returns a Logical
representing whether x
is greater than y
.
Given a Logical Bool
and two Orderables A
and B
with a common embedding, the signature is \( \mathrm{greater} : A \times B \to Bool \).
x,y  Two objects to compare. 

constexpr 
#include <boost/hana/fwd/greater_equal.hpp>
Returns a Logical
representing whether x
is greater than or equal to y
.
Given a Logical Bool
and two Orderables A
and B
with a common embedding, the signature is \( \mathrm{greater\_equal} : A \times B \to Bool \).
x,y  Two objects to compare. 

constexpr 
#include <boost/hana/fwd/less.hpp>
Returns a Logical
representing whether x
is less than y
.
Given a Logical Bool
and two Orderables A
and B
with a common embedding, the signature is \( \mathrm{less} : A \times B \to Bool \).
x,y  Two objects to compare. 

constexpr 
#include <boost/hana/fwd/less_equal.hpp>
Returns a Logical
representing whether x
is less than or equal to y
.
Given a Logical Bool
and two Orderables A
and B
with a common embedding, the signature is \( \mathrm{less\_equal} : A \times B \to Bool \).
x,y  Two objects to compare. 

constexpr 
#include <boost/hana/fwd/max.hpp>
Returns the greatest of its arguments according to the less
ordering.
min
for details.

constexpr 
#include <boost/hana/fwd/min.hpp>
Returns the smallest of its arguments according to the less
ordering.
x < y
or not. If we wanted to be mathematically correct, we should probably ask that if_(cond, x, y)
returns a common data type of x
and y
, and then the behavior of min
would follow naturally. However, I'm unsure whether this is desirable because that's a big requirement.

constexpr 
#include <boost/hana/fwd/ordering.hpp>
Returns a function performing less
after applying a transformation to both arguments.
ordering
creates a total order based on the result of applying a function to some objects, which is especially useful in conjunction with algorithms that accept a custom predicate that must represent a total order.
Specifically, ordering
is such that
or, equivalently,
Orderable
concept.Given a Logical Bool
and an Orderable B
, the signature is \( \mathrm{ordering} : (A \to B) \to (A \times A \to Bool) \).