...one of the most highly
regarded and expertly designed C++ library projects in the
world.

— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards

The concept of dimensional analysis is normally presented early on in introductory physics and engineering classes as a means of determining the correctness of an equation or computation by propagating the physical measurement units of various quantities through the equation along with their numerical values. There are a number of standard unit systems in common use, the most prominent of which is the Systeme International (also known as SI or MKS (meter-kilogram-second), which was a metric predecessor to the SI system named for three of the base units on which the system is based). The SI is the only official international standard unit system and is widely utilized in science and engineering. Other common systems include the CGS (centimeter-gram-second) system and the English system still in use in some problem domains in the United States and elsewhere. In physics, there also exist a number of other systems that are in common use in specialized subdisciplines. These are collectively referred to as natural units. When quantities representing different measurables are combined, dimensional analysis provides the means of assessing the consistency of the resulting calculation. For example, the sum of two lengths is also a length, while the product of two lengths is an area, and the sum of a length and an area is undefined. The fact that the arguments to many functions (such as exp, log, etc...) must be dimensionless quantities can be easily demonstrated by examining their series expansions in the context of dimensional analysis. This library facilitates the enforcement of this type of restriction in code involving dimensioned quantities where appropriate.

In the following discussion we view dimensional analysis as an abstraction
in which an arbitrary set of units
obey the rules of a specific algebra. We will refer to a pair of a base dimension
and a rational exponent as a **fundamental dimension**,
and a list composed of an arbitrary number of fundamental dimensions as a
**composite dimension** or, simply, **dimension**. In particular, given a set of fundamental dimensions denoted by and a set of rational exponents , any possible (composite) dimension can be written as
.

Composite dimensions obey the algebraic rules for dimensional analysis. In particular, for any scalar value, , and composite dimensions and , where , we have:

Users of a dimensional analysis library should be able to specify an arbitrary
list of base dimensions to produce a composite dimension. This potentially
includes repeated tags. For example, it should be possible to express energy
as , , , or any other permutation of mass, length, and time having
aggregate exponents of 1, 2, and -2, respectively. In order to be able to perform
computations on arbitrary sets of dimensions, all composite dimensions must
be reducible to an unambiguous final composite dimension, which we will refer
to as a **reduced dimension**, for which

- fundamental dimensions are consistently ordered
- dimensions with zero exponent are elided. Note that reduced dimensions never have more than base dimensions, one for each distinct fundamental dimension, but may have fewer.

In our implementation, base dimensions are associated with tag types. As we
will ultimately represent composite dimensions as typelists, we must provide
some mechanism for sorting base dimension tags in order to make it possible
to convert an arbitrary composite dimension into a reduced dimension. For this
purpose, we assign a unique integer to each base dimension. The `base_dimension`

class
(found in `boost/units/base_dimension.hpp`

)
uses the curiously recurring template pattern (CRTP) technique to ensure that
ordinals specified for base dimensions are unique:

template<class Derived, long N> struct base_dimension { ... };

With this, we can define the base dimensions for length, mass, and time as:

/// base dimension of length struct length_base_dimension : base_dimension<length_base_dimension,1> { }; /// base dimension of mass struct mass_base_dimension : base_dimension<mass_base_dimension,2> { }; /// base dimension of time struct time_base_dimension : base_dimension<time_base_dimension,3> { };

It is important to note that the choice of order is completely arbitrary as
long as each tag has a unique enumerable value; non-unique ordinals are flagged
as errors at compile-time. Negative ordinals are reserved for use by the library.
To define composite dimensions corresponding to the base dimensions, we simply
create MPL-conformant typelists of fundamental dimensions by using the `dim`

class to encapsulate pairs of base dimensions and `static_rational`

exponents. The `make_dimension_list`

class acts as a wrapper to ensure that the resulting type is in the form of
a reduced dimension:

typedef make_dimension_list< boost::mpl::list< dim< length_base_dimension,static_rational<1> > > >::type length_dimension; typedef make_dimension_list< boost::mpl::list< dim< mass_base_dimension,static_rational<1> > > >::type mass_dimension; typedef make_dimension_list< boost::mpl::list< dim< time_base_dimension,static_rational<1> > > >::type time_dimension;

This can also be easily accomplished using a convenience typedef provided by
`base_dimension`

:

typedef length_base_dimension::dimension_type length_dimension; typedef mass_base_dimension::dimension_type mass_dimension; typedef time_base_dimension::dimension_type time_dimension;

so that the above code is identical to the full typelist definition. Composite dimensions are similarly defined via a typelist:

typedef make_dimension_list< boost::mpl::list< dim< length_base_dimension,static_rational<2> > > >::type area_dimension; typedef make_dimension_list< boost::mpl::list< dim< mass_base_dimension,static_rational<1> >, dim< length_base_dimension,static_rational<2> >, dim< time_base_dimension,static_rational<-2> > > >::type energy_dimension;

A convenience class for composite dimensions with integer powers is also provided:

typedef derived_dimension<length_base_dimension,2>::type area_dimension; typedef derived_dimension<mass_base_dimension,1, length_base_dimension,2, time_base_dimension,-2>::type energy_dimension;