C/C++ check if one bit is set in, i.e. int variable

问题

int temp = 0x5E; // in binary 0b1011110.

Is there such a way to check if bit 3 in temp is 1 or 0 without bit shifting and masking.

Just want to know if there is some built in function for this, or am I forced to write one myself.

回答1:

In C, if you want to hide bit manipulation, you can write a macro:

#define CHECK_BIT(var,pos) ((var) & (1<<(pos)))

and use it this way to check the n^th bit from the right end:

CHECK_BIT(temp, n - 1)

In C++, you can use std::bitset.

回答2:

Check if bit N (starting from 0) is set:

temp & (1 << N)

There is no builtin function for this.

回答3:

I would just use a std::bitset if it's C++. Simple. Straight-forward. No chance for stupid errors.

typedef std::bitset<sizeof(int)> IntBits;
bool is_set = IntBits(value).test(position);

or how about this silliness

template<unsigned int Exp>
struct pow_2 {
    static const unsigned int value = 2 * pow_2<Exp-1>::value;
};

template<>
struct pow_2<0> {
    static const unsigned int value = 1;
};

template<unsigned int Pos>
bool is_bit_set(unsigned int value)
{
    return (value & pow_2<Pos>::value) != 0;
} 

bool result = is_bit_set<2>(value);

回答4:

What the selected answer is doing is actually wrong. The below function will return the bit position or 0 depending on if the bit is actually enabled. This is not what the poster was asking for.

#define CHECK_BIT(var,pos) ((var) & (1<<(pos)))

Here is what the poster was originally looking for. The below function will return either a 1 or 0 if the bit is enabled and not the position.

#define CHECK_BIT(var,pos) (((var)>>(pos)) & 1)

回答5:

According to this description of bit-fields, there is a method for defining and accessing fields directly. The example in this entry goes:

struct preferences {
    unsigned int likes_ice_cream : 1;
    unsigned int plays_golf : 1;
    unsigned int watches_tv : 1;
    unsigned int reads_books : 1;
}; 

struct preferences fred;

fred.likes_ice_cream = 1;
fred.plays_golf = 1;
fred.watches_tv = 1;
fred.reads_books = 0;

if (fred.likes_ice_cream == 1)
    /* ... */

Also, there is a warning there:

However, bit members in structs have practical drawbacks. First, the ordering of bits in memory is architecture dependent and memory padding rules varies from compiler to compiler. In addition, many popular compilers generate inefficient code for reading and writing bit members, and there are potentially severe thread safety issues relating to bit fields (especially on multiprocessor systems) due to the fact that most machines cannot manipulate arbitrary sets of bits in memory, but must instead load and store whole words.

回答6:

Yeah, I know I don't "have" to do it this way. But I usually write:

    /* Return type (8/16/32/64 int size) is specified by argument size. */
template<class TYPE> inline TYPE BIT(const TYPE & x)
{ return TYPE(1) << x; }

template<class TYPE> inline bool IsBitSet(const TYPE & x, const TYPE & y)
{ return 0 != (x & y); }

E.g.:

IsBitSet( foo, BIT(3) | BIT(6) );  // Checks if Bit 3 OR 6 is set.

Amongst other things, this approach:

Accommodates 8/16/32/64 bit integers.
Detects IsBitSet(int32,int64) calls without my knowledge & consent.
Inlined Template, so no function calling overhead.
const& references, so nothing needs to be duplicated/copied. And we are guaranteed that the compiler will pick up any typo's that attempt to change the arguments.
0!= makes the code more clear & obvious. The primary point to writing code is always to communicate clearly and efficiently with other programmers, including those of lesser skill.
While not applicable to this particular case... In general, templated functions avoid the issue of evaluating arguments multiple times. A known problem with some #define macros.
E.g.: #define ABS(X) (((X)<0) ? - (X) : (X))
ABS(i++);

回答7:

You can use a Bitset - http://www.cppreference.com/wiki/stl/bitset/start.

回答8:

Use std::bitset

#include <bitset>
#include <iostream>

int main()
{
    int temp = 0x5E;
    std::bitset<sizeof(int)*CHAR_BITS>   bits(temp);

    // 0 -> bit 1
    // 2 -> bit 3
    std::cout << bits[2] << std::endl;
}

回答9:

There is, namely the _bittest intrinsic instruction.

回答10:

I use this:

#define CHECK_BIT(var,pos) ( (((var) & (pos)) > 0 ) ? (1) : (0) )

where "pos" is defined as 2^n (i.g. 1,2,4,8,16,32 ...)

Returns: 1 if true 0 if false

回答11:

i was trying to read a 32-bit integer which defined the flags for an object in PDFs and this wasn't working for me

what fixed it was changing the define:

#define CHECK_BIT(var,pos) ((var & (1 << pos)) == (1 << pos))

the operand & returns an integer with the flags that both have in 1, and it wasn't casting properly into boolean, this did the trick

回答12:

You could "simulate" shifting and masking: if((0x5e/(2*2*2))%2) ...

回答13:

For the low-level x86 specific solution use the x86 TEST opcode.

Your compiler should turn _bittest into this though...

回答14:

Why not use something as simple as this?

uint8_t status = 255;
cout << "binary: ";

for (int i=((sizeof(status)*8)-1); i>-1; i--)
{
  if ((status & (1 << i)))
  {
    cout << "1";
  } 
  else
  {
    cout << "0";
  }
}

OUTPUT: binary: 11111111

回答15:

if you just want a real hard coded way:

 #define IS_BIT3_SET(var) ( ((var) & 0x04) == 0x04 )

note this hw dependent and assumes this bit order 7654 3210 and var is 8 bit.

#include "stdafx.h"
#define IS_BIT3_SET(var) ( ((var) & 0x04) == 0x04 )
int _tmain(int argc, _TCHAR* argv[])
{
    int temp =0x5E;
    printf(" %d \n", IS_BIT3_SET(temp));
    temp = 0x00;
    printf(" %d \n", IS_BIT3_SET(temp));
    temp = 0x04;
    printf(" %d \n", IS_BIT3_SET(temp));
    temp = 0xfb;
    printf(" %d \n", IS_BIT3_SET(temp));
    scanf("waitng %d",&temp);

    return 0;
}

Results in:

1 0 1 0

回答16:

While it is quite late to answer now, there is a simple way one could find if Nth bit is set or not, simply using POWER and MODULUS mathematical operators.

Let us say we want to know if 'temp' has Nth bit set or not. The following boolean expression will give true if bit is set, 0 otherwise.

( temp MODULUS 2^N+1 >= 2^N )

Consider the following example:

int temp = 0x5E; // in binary 0b1011110 // BIT 0 is LSB

If I want to know if 3rd bit is set or not, I get

(94 MODULUS 16) = 14 > 2^3

So expression returns true, indicating 3rd bit is set.

回答17:

One approach will be checking within the following condition:

if ( (mask >> bit ) & 1)

An explanation program will be:

#include <stdio.h>

unsigned int bitCheck(unsigned int mask, int pin);

int main(void){
   unsigned int mask = 6;  // 6 = 0110
   int pin0 = 0;
   int pin1 = 1;
   int pin2 = 2;
   int pin3 = 3;
   unsigned int bit0= bitCheck( mask, pin0);
   unsigned int bit1= bitCheck( mask, pin1);
   unsigned int bit2= bitCheck( mask, pin2);
   unsigned int bit3= bitCheck( mask, pin3);

   printf("Mask = %d ==>>  0110\n", mask);

   if ( bit0 == 1 ){
      printf("Pin %d is Set\n", pin0);
   }else{
      printf("Pin %d is not Set\n", pin0);
   }

    if ( bit1 == 1 ){
      printf("Pin %d is Set\n", pin1);
   }else{
      printf("Pin %d is not Set\n", pin1);
   }

   if ( bit2 == 1 ){
      printf("Pin %d is Set\n", pin2);
   }else{
      printf("Pin %d is not Set\n", pin2);
   }

   if ( bit3 == 1 ){
      printf("Pin %d is Set\n", pin3);
   }else{
      printf("Pin %d is not Set\n", pin3);
   }
}

unsigned int bitCheck(unsigned int mask, int bit){
   if ( (mask >> bit ) & 1){
      return 1;
   }else{
      return 0;
   }
}

Output:

Mask = 6 ==>>  0110
Pin 0 is not Set
Pin 1 is Set
Pin 2 is Set
Pin 3 is not Set

回答18:

the fastest way seems to be a lookup table for masks

来源：https://stackoverflow.com/questions/523724/c-c-check-if-one-bit-is-set-in-i-e-int-variable

标签

c++

bit-manipulation