Re: [Boost-users] help with integral type conversion trait

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-07-23 17:54:18

On Jul 23, 2005, at 1:48 PM, Slawomir Lisznianski wrote:

> Hello,
>
> I'm looking for a way to convert an integral type into its unsigned
> equivalent. For example, assume a trait called to_unsigned_integral 
> such
> that:
>
> to_unsigned_integral<int>::type   ---> yields unsigned int
> to_unsigned_integral<short>::type   ---> yields unsigned short
> to_unsigned_integral<unsigned char>::type   ---> yields unsigned char
> ...

template <class T> struct to_unsigned_integral
     {typedef T type;};
template <> struct to_unsigned_integral<char>
     {typedef unsigned char type;};
template <> struct to_unsigned_integral<signed char>
     {typedef unsigned char type;};
template <> struct to_unsigned_integral<short>
     {typedef unsigned short type;};
template <> struct to_unsigned_integral<int>
     {typedef unsigned int type;};
template <> struct to_unsigned_integral<long>
     {typedef unsigned long type;};
#if YOUVE_GOT_IT
template <> struct to_unsigned_integral<long long>
     {typedef unsigned long long type;};
#endif

I've found it pretty useful myself too. :-)

If you're paranoid about non-integral types you could dress up the 
primary template a little to return T only for integral types, and void 
otherwise (or static_assert).  But in practice I haven't come across 
the need to go to that trouble.

This tool comes in very handy when doing bit manipulations on generic 
types, e.g.:

template <class T>
inline
T
rotate_left(T x, int n = 1);

(bit manipulations are more portable if you traffic in unsigned types)

-Howard