Base units are defined much like base dimensions.
基礎單位(base unit)定義類似於基礎量綱(base dimension)。

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

Again negative ordinals are reserved.
負數的序號再次被保留。

As an example, in the following we will implement a subset of the SI unit system based on the fundamental dimensions given above, demonstrating all steps necessary for a completely functional system. First, we simply define a unit system that includes type definitions for commonly used units:
舉個例子，下面我們將根據之前定義的基本量綱(fundamental dimension)實現一SI單位系統(SI unit system)的一部分， 展示定義一個完整系統所必須的步驟。首先，我們簡單地定義一個單位系統(unit system)包含常見單位的類型定義(type definition)：

struct meter_base_unit : base_unit<meter_base_unit, length_dimension, 1> { };
struct kilogram_base_unit : base_unit<kilogram_base_unit, mass_dimension, 2> { };
struct second_base_unit : base_unit<second_base_unit, time_dimension, 3> { };

typedef make_system<
    meter_base_unit,
    kilogram_base_unit,
    second_base_unit>::type mks_system;

/// unit typedefs
typedef unit<dimensionless_type,mks_system>      dimensionless;

typedef unit<length_dimension,mks_system>        length;
typedef unit<mass_dimension,mks_system>          mass;
typedef unit<time_dimension,mks_system>          time;

typedef unit<area_dimension,mks_system>          area;
typedef unit<energy_dimension,mks_system>        energy;

The macro BOOST_UNITS_STATIC_CONSTANT is provided in boost/units/static_constant.hpp to facilitate ODR- and thread-safe constant definition in header files. We then define some constants for the supported units to simplify variable definitions:
宏BOOST_UNITS_STATIC_CONSTANT定義於boost/units/static_constant.hpp頭文件中，以方便地在頭文件中實現ODR-(One Definition Rule 一次定義法則)和線程安全(thread-safe)的常量(constant)定義。 我們接下來定義幾個支持單位的常量來簡化變量定義：

/// unit constants 
BOOST_UNITS_STATIC_CONSTANT(meter,length);
BOOST_UNITS_STATIC_CONSTANT(meters,length);
BOOST_UNITS_STATIC_CONSTANT(kilogram,mass);
BOOST_UNITS_STATIC_CONSTANT(kilograms,mass);
BOOST_UNITS_STATIC_CONSTANT(second,time);
BOOST_UNITS_STATIC_CONSTANT(seconds,time);

BOOST_UNITS_STATIC_CONSTANT(square_meter,area);
BOOST_UNITS_STATIC_CONSTANT(square_meters,area);
BOOST_UNITS_STATIC_CONSTANT(joule,energy);
BOOST_UNITS_STATIC_CONSTANT(joules,energy);

If support for textual output of units is desired, we can also specialize the base_unit_info class for each fundamental dimension tag:
如果支持單位(unit)的文本輸出(textual output)，我們需要為每個基本量綱標籤(fundamental dimension tag)特化(specialize)base_unit_info類：

template<> struct base_unit_info<test::meter_base_unit>
{
    static std::string name()               { return "meter"; }
    static std::string symbol()             { return "m"; }
};

and similarly for kilogram_base_unit and second_base_unit. A future version of the library will provide a more flexible system allowing for internationalization through a facet/locale-type mechanism. The name() and symbol() methods of base_unit_info provide full and short names for the base unit. With these definitions, we have the rudimentary beginnings of our unit system, which can be used to determine reduced dimensions for arbitrary unit calculations.
kilogram_base_unit和second_base_unit也是同樣。 未來的版本會通過facet/locale-type機制提供一個支持國際化(internationalization)的可伸縮(flexible)系統。 base_unit_info類的name()和symbol()方法提供了基礎單位(base unit)的全稱和簡稱。 通過這些定義，我們有了單位系統(unit system)最基本的開始，能夠為任意的單位運算提供reduced dimension。

Now, it is also possible to define a base unit as being a multiple of another base unit. For example, the way that kilogram_base_unit is actually defined by the library is along the following lines
現在，我們有可能定義一個基礎單位是另一個基礎單位的倍數。例如，kilogram_base_unit在庫中就是下面這樣定義的

struct gram_base_unit : boost::units::base_unit<gram_base_unit, mass_dimension, 1> {};
typedef scaled_base_unit<gram_base_unit, scale<10, static_rational<3> > > kilogram_base_unit;

This basically defines a kilogram as being 10^3 times a gram.
這裡定義了千克(kilogram)是克(gram)的1000倍。

There are several advantages to this approach.
這種做法有幾種好處。

We can also scale a unit as a whole, rather than scaling the individual base units which comprise it. For this purpose, we use the metafunction make_scaled_unit. The main motivation for this feature is the metric prefixes defined in boost/units/systems/si/prefixes.hpp.
我們也可以直接為單位(unit)定義比例，而不是為組合的每個基礎單位(base unit)定義比例。 為了這個目的，我們可以使用元函數(metafunction) make_scaled_unit。 這個特點主要的動機是為了定義在boost/units/systems/si/prefixes.hpp文件中的公制前綴(metric prefixes)。

A simple example of its usage would be.
一個用法的簡單例子如下。

typedef make_scaled_unit<si::time, scale<10, static_rational<-9> > >::type nanosecond;

nanosecond is a specialization of unit, and can be used in a quantity normally.
納秒(nanosecond)是一個特化(specialization)的單位(unit)，可以像通常一樣表示數量(quantity)。

quantity<nanosecond> t(1.0 * si::seconds);
std::cout << t << std::endl;    // prints 1e9 ns