如何将boost::units::metric::hour_base_unit转换为boost::units::si::time？

Question

我想让下面的代码正确编译：

#include <boost/units/systems/si.hpp>
#include <boost/units/base_units/metric/hour.hpp>

int main(int argc, char **argv) {
  using namespace boost::units;
  quantity<si::time> t = 2.0 * metric::hour_base_unit::unit_type(); 
}

使用gcc，我得到了以下错误：

error: conversion from ‘boost::units::multiply_typeof_helper<double, boost::units::unit<boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::heterogeneous_system<boost::units::heterogeneous_system_impl<boost::units::list<boost::units::heterogeneous_system_dim<boost::units::scaled_base_unit<boost::units::si::second_base_unit, boost::units::scale<60l, boost::units::static_rational<2l> > >, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::dimensionless_type> >, void> >::type {aka boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::heterogeneous_system<boost::units::heterogeneous_system_impl<boost::units::list<boost::units::heterogeneous_system_dim<boost::units::scaled_base_unit<boost::units::si::second_base_unit, boost::units::scale<60l, boost::units::static_rational<2l> > >, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::dimensionless_type> >, void>, double>}’ to non-scalar type ‘boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > > > >’ requested
   quantity<si::time> t = 2.0 * metric::hour_base_unit::unit_type();
                               ^

我遗漏了什么？

Answer 1

快速浏览一下文档就会发现，unit类之间的大多数(如果不是全部)转换都是explicit (它是一个design decision)。因此，您可以使用直接初始化而不是复制初始化：

#include <boost/units/systems/si.hpp>
#include <boost/units/base_units/metric/hour.hpp>

int main(int argc, char **argv) {
  using namespace boost::units;
  quantity<si::time,double> xxxxxx (2.0 * metric::hour_base_unit::unit_type());
}

而且它是有效的。

在this answer中解释了为什么这种方法有效，而您的尝试却不起作用。

希望这能有所帮助。