C++ – Simple Way to Set/Unset an Individual Bit

bit-manipulationc++

Right now I'm using this to set/unset individual bits in a byte:

if (bit4Set)
   nbyte |= (1 << 4);
else
   nbyte &= ~(1 << 4);

But, can't you do that in a more simple/elegant way? Like setting or unsetting the bit in a single operation?

Note: I understand I can just write a function to do that, I'm just wondering if I won't be reinventing the wheel.

Best Answer

Sure! It would be more obvious if you expanded the |= and &= in your code, but you can write:

nbyte = (nbyte & ~(1<<4)) | (bit4Set<<4);

Note that bit4Set must be zero or one —not any nonzero value— for this to work.

Setting a bit

Use the bitwise OR operator (|) to set nth bit of number to 1.

// Can be whatever unsigned integer type you want, but
// it's important to use the same type everywhere to avoid
// performance issues caused by mixing integer types.
typedef unsigned long Uint;

// In C++, this can be template.
// In C11, you can make it generic with _Generic, or with macros prior to C11.
inline Uint bit_set(Uint number, Uint n) {
    return number | ((Uint)1 << n);
}

Note that it's undefined behavior to shift by more than the width of a Uint. The same applies to all remaining examples.

Clearing a bit

Use the bitwise AND operator (&) to set the nth bit of number to 0.

inline Uint bit_clear(Uint number, Uint n) {
    return number & ~((Uint)1 << n);
}

You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

Use the bitwise XOR operator (^) to toggle the nth bit of number.

inline Uint bit_toggle(Uint number, Uint n) {
    return number ^ ((Uint)1 << n);
}

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift number n to the right, then bitwise AND it:

// bool requires #include <stdbool.h> prior to C23
inline bool bit_check(Uint number, Uint n) {
    return (number >> n) & (Uint)1;
}

Changing the nth bit to x

There are alternatives with worse codegen, but the best way is to clear the bit like in bit_clear, then set the bit to value, similar to bit_set.

inline Uint bit_set_to(Uint number, Uint n, bool x) {
    return (number & ~((Uint)1 << n)) | ((Uint)x << n);
}

All solutions have been tested to provide optimal codegen with GCC and clang. See https://godbolt.org/z/Wfzh8xsjW.

C++ – How to Set Individual Bits

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