Boost C++ Libraries: Ticket #10082: implicit cpp_rational(double)

John Maddock — Fri, 30 May 2014 18:07:28 GMT

Ugh, I'm a bit conflicted by this: you're quite right that the double value is converted exactly, however making this implicit does allow you to shoot yourself in the foot, for example:

cpp_rational r(0.1);

Assigns the value 3602879701896397/36028797018963968 to r, which is probably not what was intended! Of course a naive user may just add the necessary cast and shoot themselves anyway, but on balance I think I'm marginally against making the conversion implicit.

marc.glisse@… — Sat, 31 May 2014 08:06:53 GMT

Argh, please don't base your design on dumb users. Using expression templates in the first place will cause problems for dumb users (and even not so dumb ones). And the 0.1 issue is in my opinion orthogonal to implicit/explicit. The example as you wrote it already compiles (it uses the explicit constructor), you want to write it:

cpp_rational r = 0.1;

if it is supposed to be an argument against making it implicit. But the "problem" won't really become worse by making the constructor implicit.

status changed; resolution set

John Maddock — Sun, 01 Jun 2014 17:44:55 GMT

status new → closed
resolution → fixed

Fixed in https://github.com/boostorg/multiprecision/commit/0481ca18ec39a3794f1db008bde31ac5e22e8d93