The units::constants namespace provides the physical constants as typed quantities — each carries its correct dimension, so it participates in dimensional analysis like any other quantity. Values are the 2018 CODATA recommended values. Defined in include/units.h.

#include <units.h>
using namespace units;
 
// The constants are typed quantities, so this is dimensionally checked:
auto momentum = constants::m_e * (0.01 * constants::c);   // electron mass * a velocity -> momentum

Bring them into scope with units::constants:: (or using namespace units; and constants::c).

Symbol	Constant	Value	Dimension
pi	Ratio of a circle's circumference to its diameter	3.14159265358979323846	dimensionless
c	Speed of light in vacuum	299 792 458	m/s
G	Newtonian constant of gravitation	6.67430×10⁻¹¹	m³·kg⁻¹·s⁻²
h	Planck constant	6.62607015×10⁻³⁴	J·s
h_bar	Reduced Planck constant (ℏ)	1.054571817×10⁻³⁴	J·s
mu0	Vacuum permeability (μ₀)	1.25663706212×10⁻⁶	N·A⁻²
epsilon0	Vacuum permittivity (ε₀)	8.8541878128×10⁻¹²	F·m⁻¹
Z0	Characteristic impedance of vacuum	376.730313668	Ω
k_e	Coulomb constant	8.9875517923×10⁹	N·m²·C⁻²
e	Elementary charge	1.602176634×10⁻¹⁹	C
m_e	Electron mass	9.1093837015×10⁻³¹	kg
m_p	Proton mass	1.67262192369×10⁻²⁷	kg
mu_B	Bohr magneton	9.2740100783×10⁻²⁴	J·T⁻¹
N_A	Avogadro constant	6.02214076×10²³	mol⁻¹
R	Molar gas constant	8.314462618	J·K⁻¹·mol⁻¹
k_B	Boltzmann constant	1.380649×10⁻²³	J·K⁻¹
F	Faraday constant	96 485.33212	C·mol⁻¹
sigma	Stefan–Boltzmann constant (σ)	5.670374419×10⁻⁸	W·m⁻²·K⁻⁴

Because each constant is a typed quantity, an expression that misuses one — for example adding c (a velocity) to m_e (a mass) — is a compile error, just like any other dimensional mistake (see type safety).