[boost] [multiprecision] General/design query about which conversions should be implicit/explicit

Subject: [boost] [multiprecision] General/design query about which conversions should be implicit/explicit
From: John Maddock (jz.maddock_at_[hidden])
Date: 2014-05-31 07:56:00

Folks,

I have an open bug report https://svn.boost.org/trac/boost/ticket/10082 
that requests that conversions from floating point to rational 
multiprecision-types be made implicit (currently they're explicit).

Now on the one hand the bug report is correct: these are non-lossy 
conversions, so there's no harm in them being implicit.  However, it 
still sort of feels wrong to me, the only arguments against I can come 
up with are:

1) An implicit conversion lets you assign values such as 0.1 to a 
rational (which actually leads to 3602879701896397/36028797018963968 not 
1/10), where as making the conversion explicit at least forces you to 
use a cast (or an explicit construction).
2) Floating point values can result in arbitrarily large integer parts 
to the rational, effectively running the machine out of memory. 
Arguably the converting constructor should guard against that, though 
frankly exactly how is less clear :-(

Does anyone else have any strong views or insights into this?

Thanks in advance, John.