Re: 2s complement test?

From:

"James Kanze" <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

21 Jan 2007 08:07:33 -0500

Message-ID:

<1169297628.588044.109800@m58g2000cwm.googlegroups.com>

Chris wrote:

Bjorn Fahller wrote:

Is it possible to make a compile time test if an integer type is 2s
complement or not?

It's easy to check at runtime by using numeric_limits<T>::is_signed and
numeric_limits<T>::min() == -numeric_limits<T>::min().

What I want to do is to specialize a template based on T being 2s complement
or not.

Try this:

template <typename T,
      bool isSigned = std::numeric_limits<T>::is_signed,
      bool isInteger = std::numeric_limits<T>::is_integer>
struct is_2sComp
{
    static const bool value = false;
};

template <typename T>
struct is_2sComp<T, true, true>
{
private:
    static const T one = 1;
    static const T negone = -1;

public:
    static const bool value = ((~one) + 1) == negone;
};

For types signed char and short, integral promotion means that
the actual answer corresponds to the representation of int, not
of the type in question.

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