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Scientific programmers typically make heavy use of a programming language's native numeric data types. This practice, however, obscures the diverse intentions (e.g., distances, masses, energies, momenta, etc.) that any such purely numeric value could represent.

While limitations in programming language expressiveness and compiler technology have historically made such efforts difficult, the SI Library, known as SIunits, provides, in a modern programming language (C++), a convenient means of expressing, computing with, and displaying numeric values with attached units. The fundamental requirements, (1) to apply compile-time type-checking while (2) avoiding time and space overhead at run-time, have been met. Thus, SIunits enables automated (compile-time) checking of the dimensions of the numeric values within a computer program, and so provides the well-known benefits of type-safety to a new category of computing contexts.

In particular, SIunits provides data types corresponding to all the base dimensions specified by le Systeme International d'Unites (SI), the recognized international standard (SI) for describing measurable quantities and their units. SIunits also provides a mechanism to generate additional data types to describe and represent derived dimensions in accordance with SI's provisions. Thus, for example, an object resulting from the product of two Length objects is automatically of type Area. All the base and composite units specified by SI are provided as well, as are forms of the mandated prefixes for describing multiples and sub-multiples of the units. A significant number of additional standard values (any of which may be used as units), are also furnished within the library. As additional features, the library permits user specification, at compile time, of precision, calibration, and models.