Boost.Units: Is it possible to convert on assignment without casting?
Hi. I have two unit quantities defined as follow: static const auto Hz = boost::units::si::hertz; static const auto KHz = boost::units::si::kilo * Hz; using Hertz = boost::units::quantity<decltype( Hz ), double>; using Kilohertz = boost::units::quantity<decltype( KHz ), double>; Assigning one type to another does not compile: Kilohertz freqKhz = Kilohertz::from_value( 1.234 ); Hertz freqHz = freqKhz;
error: conversion from 'Kilohertz {aka boost::units::quantity<...>, double>}' to non-scalar type 'Hertz {aka boost::units::quantity<...>, double>}' requested [1]
Is this possible, should it be enabled or is it not supported? I am using boost 01.67, Mingw 5.3 and MSVC 2017 (C++14) The following options work as expected: Hertz freq1 = static_cast<Hertz>( freqKhz ); Hertz freq2 = Hertz( freqKhz ); But this adds a bit more code to maintain and keep in sync, one thing one tries to avoid when using the library since one needs to keep the LHS and the RHS of the opertor= in sync. PS: The chrono library allows this type of conversion given there is no information lost (but for double as storage types this should not be the case) std::chrono::seconds sec = std::chrono::seconds { 10 }; std::chrono::nanoseconds nsec = sec; [1] https://gcc.godbolt.org/z/0sTHQQ Regards, Carel
On 1/14/19 12:50 AM, Carel Combrink via Boost-users wrote:
Hi.
Stephane Watanabe can likely give a better answer. But I'm going to take a shot anyhow. Note I made a presentation at CppCon 2015 https://www.youtube.com/watch?v=qphj8ZuZlPA&t=4s The slides and demo code are available at http://www.rrsd.com/software_development/boost/CPPCon2015/CPPCon2015.zip
I have two unit quantities defined as follow: static const auto Hz = boost::units::si::hertz; using Hertz = boost::units::quantity<decltype( Hz ), double>;
On the wrong track here. Make a clearer distinction between quantities, dimensions and and units. Also, I strongly recommend against using auto in cases like this. It's by explicitly specifying the types which leads one to understand where he has made an error in units/quantities/dimensions. Robert Ramey
AMDG On 1/14/19 1:50 AM, Carel Combrink via Boost-users wrote:
I have two unit quantities defined as follow: static const auto Hz = boost::units::si::hertz; static const auto KHz = boost::units::si::kilo * Hz; using Hertz = boost::units::quantity<decltype( Hz ), double>; using Kilohertz = boost::units::quantity<decltype( KHz ), double>;
Assigning one type to another does not compile: Kilohertz freqKhz = Kilohertz::from_value( 1.234 ); Hertz freqHz = freqKhz;
This is not assignment. It uses the copy constructor and requires an implicit conversion (which is forbidden by design). Try direct construction, instead: Hertz freqHz{freqKhz};
error: conversion from 'Kilohertz {aka boost::units::quantity<...>, double>}' to non-scalar type 'Hertz {aka boost::units::quantity<...>, double>}' requested [1]
Is this possible, should it be enabled or is it not supported? I am using boost 01.67, Mingw 5.3 and MSVC 2017 (C++14) <snip>
In Christ, Steven Watanabe
participants (3)
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Carel Combrink -
Robert Ramey -
Steven Watanabe