Quick Start

Before discussing the basics of the library, we first define a few terms that will be used frequently in the following :
討論庫之前，我們先定義一些後面會經常用到的術語：

Base dimension : A base dimension is loosely defined as a measurable entity of interest; in conventional dimensional analysis, base dimensions include length ([L]), mass ([M]), time ([T]), etc... but there is no specific restriction on what base dimensions can be used. Base dimensions are essentially a tag type and provide no dimensional analysis functionality themselves.
基礎量綱：基礎量綱被鬆散定義為可計量的實體；在分析量綱轉換中，基礎量綱包括長度(length [L])，質量(mass [M])，時間(time [T])等。但哪個被使用並沒有限制。基礎量綱只是必需的標籤類型，本身並不提供量綱分析功能。
Dimension : A collection of zero or more base dimensions, each potentially raised to a different rational power. For example, area = [L]^2, velocity = [L]^1/[T]^1, and energy = [M]^1 [L]^2/[T]^2 are all dimensions.
量綱：零或多個基礎量綱的集合，每個基礎量綱都有一個不同的有理數指數。例如，面積(area) = [L]^2，速度(velocity) = [L]^1/[T]^1，能量 (energy) = [M]^1 [L]^2/[T]^2，這些都是dimension。
Base unit : A base unit represents a specific measure of a dimension. For example, while length is an abstract measure of distance, the meter is a concrete base unit of distance. Conversions are defined using base units. Much like base dimensions, base units are a tag type used solely to define units and do not support dimensional analysis algebra.
基礎單位：一個基礎單位表示一個量綱的特定量度。例如，長度是距離的一個抽像的度量，而米是距離的一個具體的基礎單位。轉換使用基礎單位進行定義。就像基礎量綱(base dimension)一樣，基礎單位也是一個標籤類型，用來獨立地定義單位，並且也不提供量綱的代數分析(???)。
Unit : A set of base units raised to rational exponents, e.g. kg^1 m^1/s^2.
單位：一系列基礎單位和有理數指數的集合，例如，kg^1 m^1/s^2。
System : A unit system is a collection of base units representing all the measurable entities of interest for a specific problem. For example, the SI unit system defines seven base units : length ([L]) in meters, mass ([M]) in kilograms, time ([T]) in seconds, current ([I]) in amperes, temperature ([theta]) in kelvin, amount ([N]) in moles, and luminous intensity ([J]) in candelas. All measurable entities within the SI system can be represented as products of various integer or rational powers of these seven base units.
系統：一個單位系統是可以表達特定問題所有可度量實體的基礎單位的集合。例如，SI單位系統定義了七個基礎單位：長度(length [L])使用米(meters)，質量(mass [M])使用千克(kilograms)，時間(time [T])使用秒(seconds)，電流(current [I]) 使用安培(amperes)，溫度(temperature [theta])使用開爾文(kelvin)，數量(amount [N])使用摩爾(moles)，發光強度(luminous intensity [J])使用堪(candelas)。SI系統的所有可計量實體都可以用這七個基礎單位的不同有理數次方表示。
Quantity : A quantity represents a concrete amount of a unit. Thus, while the meter is the base unit of length in the SI system, 5.5 meters is a quantity of length in that system.
量：量表示一個單位的具體數量。也就是說，SI系統中米是長度的基礎單位，5.5米是長度的量。

To begin, we present two short tutorials. Tutorial1 demonstrates the use of SI units. After including the appropriate system headers and the headers for the various SI units we will need (all SI units can be included with boost/units/systems/si.hpp) and for quantity I/O (boost/units/io.hpp), we define a function that computes the work, in joules, done by exerting a force in newtons over a specified distance in meters and outputs the result to std::cout. The quantity class accepts a second template parameter as its value type; this parameter defaults to double if not otherwise specified. To demonstrate the ease of using user-defined types in dimensional calculations, we also present code for computing the complex impedance using std::complex<double> as the value type :
一上來，我們展示兩個很短的tutorials。Tutorial1 展示了對SI單位的使用。包含了合適的系統和需要用的SI單位的頭文件(所有的SI單位包含在boost/units/systems/si.hpp)和量的I/O (boost/units/io.hpp)，我們定義了一個函數計算功(work)，單位是焦耳(joules)，通過施加力(work, newtons)作用一段距離(distance, meters) 並且將結果輸出到std::cout。quantity類第二個模板參數作為數值的類型；這個參數默認是double。為了展示在量綱計算中很容易使用用戶自定義類型，我們使用std::complex<double>作為計算複數電阻(complex impedance)的數值類型：

#include <complex>
#include <iostream>

#include <boost/typeof/std/complex.hpp>

#include <boost/units/systems/si/energy.hpp>
#include <boost/units/systems/si/force.hpp>
#include <boost/units/systems/si/length.hpp>
#include <boost/units/systems/si/electric_potential.hpp>
#include <boost/units/systems/si/current.hpp>
#include <boost/units/systems/si/resistance.hpp>
#include <boost/units/systems/si/io.hpp>

using namespace boost::units;
using namespace boost::units::si;

quantity<energy> 
work(const quantity<force>& F,const quantity<length>& dx)
{
    return F*dx;
}

int main()
{   
    /// test calcuation of work
    quantity<force>     F(2.0*newton);
    quantity<length>    dx(2.0*meter);
    quantity<energy>    E(work(F,dx));
    
    std::cout << "F  = " << F << std::endl
              << "dx = " << dx << std::endl
              << "E  = " << E << std::endl
              << std::endl;

    /// check complex quantities
    typedef std::complex<double>    complex_type;
    
    quantity<electric_potential,complex_type> v = complex_type(12.5,0.0)*volts;
    quantity<current,complex_type>            i = complex_type(3.0,4.0)*amperes;
    quantity<resistance,complex_type>         z = complex_type(1.5,-2.0)*ohms;
    
    std::cout << "V   = " << v << std::endl
              << "I   = " << i << std::endl
              << "Z   = " << z << std::endl
              << "I*Z = " << i*z << std::endl
              << "I*Z == V? " << std::boolalpha << (i*z == v) << std::endl
              << std::endl;

    return 0;
}

The intent and function of the above code should be obvious; the output produced is :
上面代碼的功能和作用是很明顯的；輸出如下：

F  = 2 N
dx = 2 m
E  = 4 J

V   = (12.5,0) V
I   = (3,4) A
Z   = (1.5,-2) Ohm
I*Z = (12.5,0) V
I*Z == V? true

While this library attempts to make simple dimensional computations easy to code, it is in no way tied to any particular unit system (SI or otherwise). Instead, it provides a highly flexible compile-time system for dimensional analysis, supporting arbitrary collections of base dimensions, rational powers of units, and explicit quantity conversions. It accomplishes all of this via template metaprogramming techniques. With modern optimizing compilers, this results in zero runtime overhead for quantity computations relative to the same code without unit checking.
雖然Boost.Units庫試圖使得量綱計算簡單，但並不會綁定到某個特定的單位系統(SI或其它)。相反，它提供了一個高度可伸縮的量綱分析的編譯時系統，支持任意的基礎量綱，有理數次方單位，顯式的量值轉換集合。它通過使用模板元編程(template metaprogramming techniques) 技術完成這一切。通過適當的優化，和沒有單位檢查比較，不會帶來任何的運行時開銷。