值位作为编译时常量！ [英] Value bits as compile-time constant!

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本文介绍了值位作为编译时常量！的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

在comp.lang.c上，Hallvard B Furuseth设计了一个编译时常量

，表示无符号

整数类型中的值表示位数。在真正的C方式中，这里它是一个宏：

/ * inttype_MAX或任何（1 <<< k）-1中的位数，其中0 <= k <1。 3.2E +

10 * /

#define IMAX_BITS（m）（（m）/（（m）％0x3fffffffL + 1）/ 0x3fffffffL％0x3fffffffL

* 30 \

+（m）％0x3fffffffL /（（m）％31 + 1）/ 31％31 * 5 + 4-12 /（（m）％

31 + 3））

你为它提供一个无符号整数值，其位模式包含所有1'的
，例如：

1（十进制：1）

11（十进制：3）

111（十进制：7）

11111111（十进制：255）

要找出无符号整数类型中的值位数，你需要为它提供该类型的最大值（b $ b）即所有1'的位模式。获得无符号整数类型最大值的最简单方法是

为它分配-1：

unsigned const max_value = -1;

unsigned const amount_value_bits = IMAX_BITS（max_value）;

这里是我写的一些模板代码，它使用了它：

模板< class T>

struct IMaxBits {

模板< T m>

struct AllOnes {

static unsigned const val =

m /（m％0x3fffffffL + 1）/ 0x3fffffffL％0x3fffffffL * 30

+ m％0x3fffffffL /（m％31 + 1）/ 31％31 * 5 + 4-12 /（m％31 + 3）;

};

};

模板< class T>

struct AmountValueBits {

static unsigned const val = IMaxBits< T> ::模板AllOnes< -1> :: val;

};

#include< cstdlib>

#include< cstring>

#include< cstddef>

#include< cassert>

模板< std :: size_t width>

inline const char * CenterHoriz（const char * const p）

{

使用std :: memset;

使用std :: strlen;

使用std :: size_t;

static char spaces [width + 1];

memset（spaces，''''，width）;

size_t const len = strlen（p）;

断言（宽度> = len）;

char * const pos = spaces +（width / 2 - len / 2）;

memcpy（pos，p，len）;

返回空格;

}

＃ include< iostream>

extern char const str_uchar [] =" unsigned char" ;;

extern char const str_ushort [] =" unsigned short" ;

extern char const str_uint [] =" unsigned int";

extern char const str_ulong [] =" unsigned long";

模板< class T，const char * str>

void PrintRow（）

{

usi ng std :: cout;

unsigned const total_bits = sizeof（T）* AmountValueBits< unsigned

char> :: val;

unsigned const val_bits = AmountValueBits< T> :: val;

unsigned const pad_bits = total_bits - val_bits;

const char * const spaces_val =

val_bits> 99？ "" ：（val_bits> 9？""：""）;

const char * const spaces_pad =

pad_bits> 99？ "" ：（pad_bits> 9？""：""）;

const char * const spaces_total =

total_bits> 99？ "" ：（total_bits> 9？""：""）;

cout<< " ||" << CenterHoriz< 21>（str）<< " ||

<< spaces_val<< val_bits<< " || " << spaces_pad<<

pad_bits

<< " || " << spaces_total<< total_bits<< " || \ n"

" ------------------------------ ---------------------------------------

\ n" ;;

}

int main（）

{

使用std :: cout;

cout<<

"

=========== =================================== \ n"

" ||值位||填充位||总比特

|| \ n"

" ==================== ============================= ====================

\ n" ;;

PrintRow< unsigned char，str_uchar>（）;

PrintRow< unsigned short，str_ushort>（）;

PrintRow< unsigned，str_uint>（）;

PrintRow< unsigned long，str_ulong>（）;

cout << \ n \ n \\ nn \\ n;

std :: system（" PAUSE"）;

}

我有兴趣听听是否有人得到一些有趣的信息。价值。

-

Frederick Gotham

Over on comp.lang.c, Hallvard B Furuseth devised a compile-time constant
representing the amount of value representation bits in an unsigned
integer type. In true C fashion, here it is as a macro:
/* Number of bits in inttype_MAX, or in any (1<<k)-1 where 0 <= k < 3.2E+
10 */
#define IMAX_BITS(m) ((m) /((m)%0x3fffffffL+1) /0x3fffffffL %0x3fffffffL
*30 \
+ (m)%0x3fffffffL /((m)%31+1)/31%31*5 + 4-12/((m)%
31+3))
You supply it with an unsigned integer value whose bit-pattern consists
of all 1''s, e.g.:

1 (Decimal: 1)
11 (Decimal: 3)
111 (Decimal: 7)
11111111 (Decimal: 255)
To find out the amount of value bits in an unsigned integer type, you
supply it with that type''s max value (i.e. a bit pattern of all 1''s). The
handiest way to get the max value of an unsigned integer type is to
assign -1 to it:

unsigned const max_value = -1;

unsigned const amount_value_bits = IMAX_BITS( max_value );
Here''s some template code I''ve written which makes use of it:
template<class T>
struct IMaxBits {

template<T m>
struct AllOnes {

static unsigned const val =
m /(m%0x3fffffffL+1) /0x3fffffffL %0x3fffffffL *30
+ m%0x3fffffffL /(m%31+1)/31%31*5 + 4-12/(m%31+3);

};

};
template<class T>
struct AmountValueBits {

static unsigned const val = IMaxBits<T>::template AllOnes<-1>::val;

};
#include <cstdlib>
#include <cstring>
#include <cstddef>
#include <cassert>

template<std::size_t width>
inline const char* CenterHoriz( const char * const p )
{
using std::memset;
using std::strlen;
using std::size_t;

static char spaces[width + 1];
memset( spaces, '' '', width );

size_t const len = strlen( p );

assert( width >= len );
char * const pos = spaces + ( width / 2 - len / 2 );
memcpy( pos, p, len );

return spaces;
}

#include <iostream>

extern char const str_uchar[] = "unsigned char";
extern char const str_ushort[] = "unsigned short";
extern char const str_uint[] = "unsigned int";
extern char const str_ulong[] = "unsigned long";

template<class T, const char* str>
void PrintRow()
{
using std::cout;

unsigned const total_bits = sizeof(T) * AmountValueBits<unsigned
char>::val;
unsigned const val_bits = AmountValueBits<T>::val;
unsigned const pad_bits = total_bits - val_bits;

const char * const spaces_val =
val_bits > 99 ? "" : ( val_bits > 9 ? " " : " " );

const char * const spaces_pad =
pad_bits > 99 ? "" : ( pad_bits > 9 ? " " : " " );

const char * const spaces_total =
total_bits > 99 ? "" : ( total_bits > 9 ? " " : " " );
cout << "||" << CenterHoriz<21>(str) << "|| "
<< spaces_val << val_bits << " || " << spaces_pad <<
pad_bits
<< " || " << spaces_total << total_bits << " ||\n"

"---------------------------------------------------------------------
\n";
}

int main()
{
using std::cout;

cout <<

"
==============================================\n"
" || Value bits || Padding Bits || Total Bits
||\n"

"================================================= ====================
\n";

PrintRow<unsigned char, str_uchar>();
PrintRow<unsigned short, str_ushort>();
PrintRow<unsigned, str_uint>();
PrintRow<unsigned long, str_ulong>();

cout << "\n\n\n";

std::system( "PAUSE" );
}

I''d be interested to hear if anyone gets some "interesting" values.

--

Frederick Gotham

值位作为编译时常量！ [英] Value bits as compile-time constant!

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