问题描述
正如文档，可以直接从 boost :: multiprecision :: cpp_int 转换为 boost :: multiprecision :: cpp_dec_float ：
As is made clear in the Boost Multiprecision library documentation, it is straightforward to convert from a boost::multiprecision::cpp_int to a boost::multiprecision::cpp_dec_float:
// Some interconversions between number types are completely generic,
// and are always available, albeit the conversions are always explicit:
cpp_int cppi(2);
cpp_dec_float_50 df(cppi); // OK, int to float // <-- But fails with cpp_dec_float<0>!
能够从 cpp_int 一个固定宽度浮点类型（即 cpp_dec_float_50 ）希望可以从 cpp_int 到库中的任意宽度浮点类型 - 即 cpp_dec_float< 0> 。然而，这不工作;在Visual Studio 2013中转换失败，如下面的简单示例程序所示：
The ability to convert from a cpp_int to a fixed-width floating-point type (i.e., a cpp_dec_float_50) gives one hope that it might be possible to convert from a cpp_int to an arbitrary-width floating-point type in the library - i.e., a cpp_dec_float<0>. However, this doesn't work; the conversion fails for me in Visual Studio 2013, as the following simple example program demonstrates:
#include <boost/multiprecision/number.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
int main()
{
boost::multiprecision::cpp_int n{ 0 };
boost::multiprecision::cpp_dec_float<0> f{ n }; // Compile error in MSVC 2013
}
成功转换为 cpp_dec_float_50 ，如所预期的，但是如所指出的，我希望转换为任意精度浮点类型： cpp_dec_float< 0& / code>。
It does succeed to convert to cpp_dec_float_50, as expected, but as noted, I am hoping to convert to an arbitrary precision floating point type: cpp_dec_float<0>.
错误出现在内部Boost多精度代码的以下代码段中，位于文件< boost /multiprecision/detail/default_ops.hpp> ：
The error appear in the following snippet of code from the internal Boost Multiprecision code, in the file <boost/multiprecision/detail/default_ops.hpp>:
template <class R, class T>
inline bool check_in_range(const T& t)
{
// Can t fit in an R?
if(std::numeric_limits<R>::is_specialized && std::numeric_limits<R>::is_bounded
&& (t > (std::numeric_limits<R>::max)()))
return true;
return false;
}
错误讯息是：
错误C2784：
'enable_if :: result_type，detail :: expression :: result_type>，bool> （const
boost :: multiprecision :: detail :: expression
&，const
boost :: multiprecision :: detail :: expression&）'：
无法推导模板参数'const
boost :: multiprecision :: detail :: expression&'
from'const next_type'
可以将 boost :: multiprecision :: cpp_int 转换为 boost :: multiprecision :: cpp_dec_float< 0> / code>（而不是转换为具有固定小数精度的浮点类型，如 cpp_dec_float_50 ）？
（注意，在我的程序中，只有一个浮点数实例在任何时候被实例化，并且它很少更新，所以我很高兴这个实例占用大量内存，很长时间来支持真正巨大的数字。）
(Note that in my program, only one instance of the floating-point number is instantiated at any time, and it is updated infrequently, so I am fine with having this one instance take up lots of memory and take a long time to support really huge numbers.)
谢谢！
推荐答案
我对Boost Multiprecision没有多少经验，但在我看来，模板类 cpp_dec_float<> 是他们称为后端，您需要将它包装在数字<> 适配器中，以便将其用作算术类型。
I don't have much experience with Boost Multiprecision, but it seems to me that the template class cpp_dec_float<> is what they call a backend, and you need to wrap it in a number<> adaptor in order to use it as an arithmetic type.
以下是我对此的介绍：
Here's my take on it: Live On Coliru
#include <boost/multiprecision/number.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <iostream>
namespace mp = boost::multiprecision;
int main()
{
using Int = mp::cpp_int;
// let's think of a nice large number
Int n = 1;
for (Int f = 42; f>0; --f)
n *= f;
std::cout << n << "\n\n"; // print it for vanity
// let's convert it to cpp_dec_float
// and... do something with it
using Dec = mp::number<mp::cpp_dec_float<0> >;
std::cout << n.convert_to<Dec>();
}
输出：
1405006117752879898543142606244511569936384000000000
1.40501e+51
如果允许 convert_to<> ，则显式转换构造函数也可以使用，我希望：
If convert_to<> is allowed, then the explicit conversion constructor will also work, I expect:
Dec decfloat(n);
这篇关于如何从boost :: multiprecision :: cpp_int转换为cpp_dec_float< 0> （而不是cpp_dec_float_50等）？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！