Max value of an integer type?

[Frederick Gotham]

I'm writing a template, and I need to know the maximum value of a given
integer type at compile time. something like:

template<class NumType>
class Arb {
public:

static NumType const max = /* maximum value */;

};
If the template were only to be used with unsigned integer types, then
I'd do the following:

template<class NumType>
class Arb {
public:

static NumType const max = -1;

};
I need a compile-time constant which evaluates to the maximum value of an
integer type.

Any ideas?

If it weren't undefined behaviour to overflow a signed integer, I could
use a metaprogramming technique such as the following:

template<class T, bool overflow = false>
struct MaxIntVal {
private:

static T const internal = 1 + MaxIntVal<T,...

public:

static T const val = SCHAR_MAX + internal;
};
--

Frederick Gotham

[Bo Persson]

"Frederick Gotham" <fg*******@SPAM .comskrev i meddelandet
news:F9******** ***********@new s.indigo.ie...

posted:

>>
Frederick Gotham wrote:

>>template< class T,
T shift_by,
bool no_more_digits = shift_by == std::numeric_li mits
<T>::digits
>

...

Although this would probably work 99% of the time, I think
technically
that the standard does not require 2s compliment arithmetic (or
whatever it is called) - ie you can't assume setting all bits to 1
gives you the largest value.

"digits" gives you the amount of value bits, excluding the sign bit.

If the sign bit (if any) is zero, and if all the rest of the bits
are 1,
then you have the max value.

This of course assumes that all the bits take part in the value
representation. Not a requirement.

You are now at 99.5% mark, I guess. :-)
There is a reason for the seemingly redundant values in
numeric_limits - implementations are allowed to have unusual values
for some of them.
Bo Persson

[Frederick Gotham]

Bo Persson posted:

This of course assumes that all the bits take part in the value
representation. Not a requirement.

18.2.1.2 numeric_limits members

static const int digits;

6 Number of radix digits that can be represented without change.
7 For built-in integer types, the number of non-sign bits in the
representation.
--

Frederick Gotham

[Rolf Magnus]

Bo Persson wrote:

>"digits" gives you the amount of value bits, excluding the sign bit.

If the sign bit (if any) is zero, and if all the rest of the bits
are 1,
then you have the max value.

This of course assumes that all the bits take part in the value
representation.

AFAICS, it doesn't. As Frederick wrote, "digits" gives you the amount of
_value bits_, not the total amount of bits.

[Alf P. Steinbach]

* Frederick Gotham:

=?utf-8?B?SGFyYWxkIHZ hbiBExLNr?= posted:

>If you can use numeric_limits: :digits, wouldn't this be simpler?

template <typename T>
struct intinfo {
static const T max = (static_cast<T> (1) <<
std::numeric_l imits<T>::digit s - 1) - 1 << 1 | 1;
};

Or are there some special cases this doesn't handle correctly?

That's brilliant. Here's the updated code:

#include<limits >

template<class T>
struct IntMax {
private:

static T const MSBOnly = static_cast<T>( 1) << std::numeric_li mits
<T>::digits - 1;

public:

static T const val = MSBOnly | MSBOnly - 1;

};

It's good fun to throw academic spanners into practical wheels, so,
where does the standard exclude a grey-code representation? ;-)

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Frederick Gotham]

Alf P. Steinbach posted:

It's good fun to throw academic spanners into practical wheels, so,
where does the standard exclude a grey-code representation? ;-)

Only caveat that comes to mind is negative zero... but that shouldn't be an
issue here.

Unless you're talking about left-shifting a signed integer type... ?
(Not quite sure what you mean by "exclude a grey-code representation" )
--

Frederick Gotham

[Alf P. Steinbach]

* Frederick Gotham:

Alf P. Steinbach posted:

>It's good fun to throw academic spanners into practical wheels, so,
where does the standard exclude a grey-code representation? ;-)

Only caveat that comes to mind is negative zero... but that shouldn't be an
issue here.

Unless you're talking about left-shifting a signed integer type... ?
(Not quite sure what you mean by "exclude a grey-code representation" )

Frederick, I give you also (as I gave Protoman)... WIKIPEDIA!

<url: http://en.wikipedia.or g/wiki/Grey_code>

Note the bit pattern for the highest value when using a given number of
bits.

Sorry for the typo.

Coming from a whaling country, I naturally prefer Earl Grey tea...

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Frederick Gotham]

Alf P. Steinbach posted:

Note the bit pattern for the highest value when using a given number of
bits.

I was under the assumption that C++ had to store positive values as
follows:

0: 0000
1: 0001
2: 0010
3: 0011
4: 0100
5: 0101
6: 0110
7: 0111
8: 1000
9: 1001
10: 1010
11: 1011
12: 1100
13: 1101
14: 1110
15: 1111

Are you saying that:

(A) The machine could use another method such as "Gray code".
and
(B) My code could break on other systems, and thus isn't trully
portable?
--

Frederick Gotham

[Rolf Magnus]

Alf P. Steinbach wrote:

It's good fun to throw academic spanners into practical wheels, so,
where does the standard exclude a grey-code representation? ;-)

Hmm, I would have said 3.9.1/7, but after reading it again, it seems that
this actually doesn't prohibit a grey-code representation.

[Rolf Magnus]

Rolf Magnus wrote:

Alf P. Steinbach wrote:

>It's good fun to throw academic spanners into practical wheels, so,
where does the standard exclude a grey-code representation? ;-)

Hmm, I would have said 3.9.1/7, but after reading it again, it seems that
this actually doesn't prohibit a grey-code representation.

Ok, now I have one: 5.8/2.

[Frederick Gotham]

Rolf Magnus posted:

Ok, now I have one: 5.8/2.

The value of E1 << E2 is E1 (interpreted as a bit pattern) left-shifted E2
bit positions; vacated bits are zero-filled. If E1 has an unsigned type,
the value of the result is E1 multiplied by the quantity 2 raised to the
power E2, reduced modulo ULONG_MAX+1 if E1 has type unsigned long,
UINT_MAX+1 otherwise.
Is it the "vacated bits are zero-filled" part that you're talking about?

--

Frederick Gotham