CODEWITHSUNDEEP
c++bit-manipulation
JB_SO
JB_SO

Reputation: 491

Set individual bit in C++

I have a 5 byte data element and I need some help in figuring out how in C++ to set an individual bit of one of these byte; Please see my sample code below:

char m_TxBuf[4];

I would like to set bit 2 to high of byte m_TxBuf[1].

    
00000 0 00
      ^ This one

Any support is greatly appreciated; Thanks!

Upvotes: 4

Views: 14261

Answers (6)

Matthew Flaschen
Matthew Flaschen

Reputation: 285057

m_TxBuf[1] |= 4;

To set a bit, you use bitwise or. The above uses compound assignment, which means the left side is one of the inputs and the output.

Upvotes: 3

stinky472
stinky472

Reputation: 6797

Typically we set bits using bitwise operator OR (operator| or operator|= as a shorthand).

Assuming 8-bits to a byte (where the MSB is considered the '7st' bit and the LSB considered the 0th: MSB 0) for simplicity:

char some_char = 0;
some_char |= 1 << 0; // set the 7th bit (least significant bit)
some_char |= 1 << 1; // set the 6th bit
some_char |= 1 << 2; // set the 5th bit
// etc.

We can write a simple function:

void set_bit(char& ch, unsigned int pos)
{
    ch |= 1 << pos;
}

We can likewise test bits using operator&.

// If the 5th bit is set...
if (some_char & 1 << 2)
    ...

You should also consider std::bitset for this purpose which will make your life easier.

Upvotes: 2

Edward Strange
Edward Strange

Reputation: 40895

Just use std::bitset<40> and then index bits directly.

Upvotes: 0

Andras Vass
Andras Vass

Reputation: 11638

Bitwise operators in C++.

"...set bit 2..."

Bit endianness.

I would like to set bit 2 to high of byte m_TxBuf[1];

m_TxBuf[1] |= 1 << 2

Upvotes: 13

Amardeep AC9MF
Amardeep AC9MF

Reputation: 19064

int bitPos = 2;  // bit position to set
m_TxBuf[1] |= (1 << bitPos);

Upvotes: 3

Anon.
Anon.

Reputation: 60043

You can use bitwise-or (|) to set individual bits, and bitwise-and (&) to clear them.

Upvotes: 4

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