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Front Page / Tutorial: Metafunctions and Higher-Order Metaprogramming / Dimensional Analysis / Representing Quantities |
The types listed above are still pure metadata; to typecheck real computations we'll need to somehow bind them to our runtime data. A simple numeric value wrapper, parameterized on the number type T and on its dimensions, fits the bill:
template <class T, class Dimensions> struct quantity { explicit quantity(T x) : m_value(x) {} T value() const { return m_value; } private: T m_value; };
Now we have a way to represent numbers associated with dimensions. For instance, we can say:
quantity<float,length> l( 1.0f ); quantity<float,mass> m( 2.0f );
Note that Dimensions doesn't appear anywhere in the definition of quantity outside the template parameter list; its only role is to ensure that l and m have different types. Because they do, we cannot make the mistake of assigning a length to a mass:
m = l; // compile-time type error