Casting boost::units::quantity to double

Question

I need to pass the value of a quantity to a library for evaluation. The boost units library takes double values in SI, so the boost units library is very appealing in ensuring that requirement. However, how should I cast the quantity to a double value? The documentation and example seems to avoid this since the intent is to, rightfully so, maintain the units.

Something like:

quantity<pressure> p(101.1 * kilo * pascals);
double dblP = static_cast<double>(p);  // double value in Pascals

Going through the headers suggests... Is that the correct way to cast to the base type?

p.value();

"Going through the headers suggests... `p.value()`" So your question is? — R. Martinho Fernandes, Dec 18 '12 at 16:51
Good point - sorry I added that after the question was first asked. What I want to ensure is .value() does return what I expect reliably. Ie, is it after any needed conversions such as psi to Pa? — ccook, Dec 18 '12 at 16:59

score 9 · Answer 1 · answered Nov 11 '13 at 22:19

I think what you're looking for is this:

quantity<pressure> p(101.1 * kilo * pascals);
double dblP = p / pascals;  // double value in Pascals

If you divide out the unit, you are left with a quantity<dimensionless> which will implicitly convert to a double. This eliminates any question of what the internal representation (which value() returns) units are.

score 7 · Answer 2 · answered Feb 09 '14 at 12:59

Just spotted this. I think the intended method is to use Boost's quantity_cast operator.

quantity<pressure> p(101.1 * kilo * pascals);
double dblP = quantity_cast<double>(p);

http://www.boost.org/doc/libs/1_55_0/doc/html/boost_units/Quantities.html#boost_units.Quantities.Quantity_Construction_and_Conversion

score 5 · Accepted Answer · edited Aug 18 '17 at 08:03

5

The reference documentation shows that either implicit casts or the value() member method can be used.

operator value_type() const;

implicit conversion to value_type is allowed

const value_type & value() const;

constant accessor to value

edited Aug 18 '17 at 08:03

Louis Langholtz

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answered Dec 18 '12 at 16:54

R. Martinho Fernandes

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Thank you, that reference is very helpful (and hard to find with keywords quantity unit boost cast) – ccook Dec 18 '12 at 17:02
`operator value_type() const;` only exists for dimensionless quantites. Otherwise this would be possible: `double v = quantity()`. – alfC Jul 27 '15 at 00:48

Casting boost::units::quantity to double

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