2023-08-30 17:42:23 +02:00
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---
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tags:
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- Physical Constants
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- Text Formatting
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---
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# `si_constants`
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2023-08-31 18:59:10 +02:00
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!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/96hPY77h6)"
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2023-08-30 17:42:23 +02:00
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The next example presents all the seven defining constants of the SI system. We can observe
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how [Faster-than-lightspeed Constants](../framework_basics/faster_than_lightspeed_constants.md)
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work in practice.
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```cpp title="si_constants.cpp" linenums="1"
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--8<-- "example/si_constants.cpp:28:34"
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```
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As always, we start with the inclusion of all the needed header files. After that, for
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the simplicity of this example, we
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[hack the character of quantities](../framework_basics/character_of_a_quantity.md#hacking-the-character)
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to be able to express vector quantities with simple scalar types.
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```cpp title="si_constants.cpp" linenums="1"
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--8<-- "example/si_constants.cpp:36:"
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```
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The main part of the example prints all of the SI-defining constants. While analyzing the output of
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this program (provided below), we can easily notice that a direct printing of the quantity provides
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just a value `1` with a proper constant symbol. This is the main power of the
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[Faster-than-lightspeed Constants](../framework_basics/faster_than_lightspeed_constants.md) feature.
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Only after we explicitly convert the unit of a quantity to proper SI units we get an actual numeric
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value of the constant.
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```text
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The seven defining constants of the SI and the seven corresponding units they define:
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- hyperfine transition frequency of Cs: 1 Δν_Cs = 9192631770 Hz
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- speed of light in vacuum: 1 c = 299792458 m/s
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- Planck constant: 1 h = 6.62607015e-34 J s
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- elementary charge: 1 e = 1.602176634e-19 C
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- Boltzmann constant: 1 k = 1.380649e-23 J/K
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- Avogadro constant: 1 N_A = 6.02214076e+23 1/mol
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- luminous efficacy: 1 K_cd = 683 lm/W
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```
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