Casting boost::units::quantity to double

问题:

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();

回答1:

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

1. operator value_type() const;

   implicit conversion to value_type is allowed

2. const value_type & value() const;

   constant accessor to value

回答2:

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.

回答3:

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