This simple code can't be compiled with the -std=c 20
option:
#include <limits>
#include <boost/multiprecision/float128.hpp>
namespace bm = boost::multiprecision;
int main()
{
auto const m = std::numeric_limits<bm::float128>::max();
}
The compilation command and its error output:
hekto@ubuntu:~$ g -std=c 20 test.cpp
In file included from test.cpp:2:
/usr/include/boost/multiprecision/float128.hpp: In instantiation of ‘static std::numeric_limits<boost::multiprecision::number<boost::multiprecision::backends::float128_backend, ExpressionTemplates> >::number_type std::numeric_limits<boost::multiprecision::number<boost::multiprecision::backends::float128_backend, ExpressionTemplates> >::max() [with boost::multiprecision::expression_template_option ExpressionTemplates = boost::multiprecision::et_off; std::numeric_limits<boost::multiprecision::number<boost::multiprecision::backends::float128_backend, ExpressionTemplates> >::number_type = boost::multiprecision::number<boost::multiprecision::backends::float128_backend, boost::multiprecision::et_off>]’:
test.cpp:8:53: required from here
/usr/include/boost/multiprecision/float128.hpp:728:55: error: could not convert ‘boost::multiprecision::quad_constants::quad_max’ from ‘const __float128’ to ‘std::numeric_limits<boost::multiprecision::number<boost::multiprecision::backends::float128_backend, boost::multiprecision::et_off> >::number_type’ {aka ‘boost::multiprecision::number<boost::multiprecision::backends::float128_backend, boost::multiprecision::et_off>’}
728 | static number_type (max)() BOOST_NOEXCEPT { return BOOST_MP_QUAD_MAX; }
| ^~~~~~~~~~~~~~~~~
| |
| const __float128
Why is that?
- OS: Xubuntu 20.04.4 LTS
- Compiler: g (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0
- Boost: 1.71.0
CodePudding user response:
The documentation states:
When compiling with gcc, you need to use the flag --std=gnu 11/14/17, as the suffix 'Q' is a GNU extension. Compilation fails with the flag --std=c 11/14/17 unless you also use -fext-numeric-literals.
So you need to specify --std=gnu 20
instead of --std=c 20
. The boost documentation is not up-to-date. The flag enables various GNU extensions, one of it being __float128
.
See example on godbolt.
The underlying reason for the compiler error is that without the --std=gnu 17
or --std=gnu 20
flag, _GLIBCXX_USE_FLOAT128
is not defined, meaning BOOST_HAS_FLOAT128
is not defined, meaning __float128
is not recognized as number_kind_floating_point
(but as number_kind_unknown
).
This means that boost::multiprecision::number
cannot be implicitly constructed from a __float128
because is_restricted_conversion<__float128, boost::multiprecision::float128_backend>::value
is true
because bm::detail::is_lossy_conversion<__float128, boost::multiprecision::float128_backend>::value
is true
, because __float128
is number_kind_unknown
instead of number_kind_floating_point
.
I.e. in short, without gnu 20
, the boost::multiprecision::float128
type is not supported.
CodePudding user response:
Directly from the documentation:
When compiling with gcc, you need to use the flag
--std=gnu 11
/14
/17
, as the suffixQ
is a GNU extension. Compilation fails with the flag--std=c 11
/14
/17
unless you also use-fext-numeric-literals
.
The same applies for c 20
/gnu 20
, the docs merely haven't yet been updated for that revision.