mp-units/docs/users_guide/examples/si_constants.md

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Physical Constants Text Formatting

si_constants

!!! example "Try it on Compiler Explorer"

The next example presents all the seven defining constants of the SI system. We can observe how Faster-than-lightspeed Constants work in practice.

--8<-- "example/si_constants.cpp:28:40"

As always, we start with the inclusion of all the needed header files. The main part of the example prints all of the SI-defining constants:

--8<-- "example/si_constants.cpp:42:"

While analyzing the output of this program (provided below), we can easily notice that a direct printing of the quantity provides just a value 1 with a proper constant symbol. This is the main power of the Faster-than-lightspeed Constants feature. Only after we explicitly convert the unit of a quantity to proper SI units we get an actual numeric value of the constant.

The seven defining constants of the SI and the seven corresponding units they define:
- hyperfine transition frequency of Cs: 1 Δν_Cs = 9192631770 Hz
- speed of light in vacuum:             1 c = 299792458 m/s
- Planck constant:                      1 h = 6.62607015e-34 J s
- elementary charge:                    1 e = 1.602176634e-19 C
- Boltzmann constant:                   1 k = 1.380649e-23 J/K
- Avogadro constant:                    1 N_A = 6.02214076e+23 1/mol
- luminous efficacy:                    1 K_cd = 683 lm/W