Plato.Constants

A light framework for constant definition.

One constant

Constants are expressed in SI and CGS unit systems as Quantitys, optionally with some description:

struct Constant {
  Quantity si;
  Quantity cgs;
  const char* description;
};

A Quantity is a light structure made of a value (as a double) and unit (as a string). It can be sliced as its value so that accessing a constant, e.g. LightYear in namespace Distance, is straightforward:

const double ly = Distance::LightYear::si;

Conversions are not implemented; this could be added later through Boost.Units.

A lot of constants

Constants can be registered, so that they are look up by name, in a ConstantRegister, which is a global constant singleton filled at compile time. This provides run time access to constants, and a few short cuts, e.g.:

const auto ly = ConstantRegister::at("Distance::LightYear");
const auto lyCsg = ConstantRegister::csg("Distance::LightYear");

A CSV-like definition

A few macros are provided to ease constant declaration and act as an abstraction layer (for example, this would ease the migration to Boost.Units):

DEFINE_PLATO_CONSTANT(group, tag, siValue, siUnit, cgsValue, cgsUnit, desc)

generates a conforming struct named tag in namespace Plato::group.

Additionally:

REGISTER_PLATO_CONSTANT(group, tag, siValue, siUnit, cgsValue, cgsUnit, desc)

registers the constant in the ConstantRegister as group::tag.

Last but not least, aliases can be defined with:

ALIAS_PLATO_CONSTANT(group, tag, alias)

Versioning in API

Function std::string Constant::version() is declared in the framework. It should be defined in the user code, e.g.:

std::string Constant::version() {
  return "1.0";
}

Using the constants

To use the constants, include constant_builder.h, and create and include some constants.h file contain made of the aforementioned macros. An example such file is provided for reference.

You can also check the example program, example.cpp which is built and run as: