Re: [Boost-users] [boost][multiprecision] compile error with gcc 4.8.4

Subject: Re: [Boost-users] [boost][multiprecision] compile error with gcc 4.8.4
From: John Maddock (jz.maddock_at_[hidden])
Date: 2016-09-17 14:42:58

On 17/09/2016 18:35, Rob Conde wrote:
> The following works with me in visual studio with toolset v120 and v140.
>
> #include <boost/multiprecision/cpp_int.hpp>
> #include <boost/multiprecision/cpp_bin_float.hpp>
> #include <iostream>
>
> typedef boost::multiprecision::int128_t int128_t;
> typedef boost::multiprecision::cpp_bin_float_quad float128_t;
>
> void main()
> {
>    float128_t floatVal("6.0");
>
>    int128_t iVal(floatVal.convert_to<int128_t>());
>
>    std::cout << iVal << std::endl;
> }
>
> If I change it to have a fractional component, then indeed I get an 
> exception...maybe that's what you mean. I personally only need the 
> above behavior (though keeping the c++ truncation semantics might make 
> sense as a long term goal...but i digress). I always floor/ceil 
> because call convert_to<int128_t>().
>
> Using boost 1.61 and gcc version  4.4.7, the above also compiles and 
> works.

OK, I see that compiles with 1.61, it does not with current master 
though (what will be 1.62).

In 1.61 the code performs a lexical_cast for the conversion - that works 
more or less by accident for small integers, but for large ones it fails 
at runtime (as an exception) as the numbers get printed in scientific 
format during the lexical_cast.  Conversion to long long works fine 
though, and probably in all the cases that your conversion to int128_t does.

As I said, I need to figure out some more consistent behaviour for 
future releases.

John.