What does "0b" and "0x" stand for when assigning binary and hex?

Question

When assigning a binary value and a hexadecimal value directly you can do it as follows (respectively):

uint8_t val1 = 0b10101;
uint8_t val2 = 0xFF;

What does the 0b and 0x mean? Specifically the 0 at the front. Can you have other values instead of 0?

Also as another curious question, what other characters can go in the place of b and x? Is there one for octal as an example?

Answer 1

What does the 0b and 0x mean?

They mean that the nuneric literal is respectively in binary and hexadecimal base.

Can you have other values instead of 0?

A numeric literal starting with a non zero digit will be a decimal literal.

Also as another curious question, what other characters can go in the place of "b" and "x"?

Besides b and x, any octal digit can go there in which case it is the most significant digit of an octal literal.

Answer 2

Any and all integer literals you can create are summarized in the C++ standard by the grammar production at [lex.icon]

integer-literal:
    binary-literal integer-suffixopt
    octal-literal integer-suffixopt
    decimal-literal integer-suffixopt
    hexadecimal-literal integer-suffixopt

binary-literal:
    0b binary-digit
    0B binary-digit
    binary-literal 'opt binary-digit

octal-literal:
    0
    octal-literal 'opt octal-digit

decimal-literal:
    nonzero-digit
    decimal-literal 'opt digit

hexadecimal-literal:
    hexadecimal-prefix hexadecimal-digit-sequence

binary-digit:
    0
    1

octal-digit: one of
    0  1  2  3  4  5  6  7

nonzero-digit: one of
    1  2  3  4  5  6  7  8  9

hexadecimal-prefix: one of
    0x  0X

hexadecimal-digit-sequence:
    hexadecimal-digit
    hexadecimal-digit-sequence 'opt hexadecimal-digit

hexadecimal-digit: one of
    0  1  2  3  4  5  6  7  8  9
    a  b  c  d  e  f
    A  B  C  D  E  F

As we can deduce from the grammar, there are four types of integer literals:

• Plain decimal, that must begin with a non-zero digit.
• Octal, any number with a leading 0 (including a plain 0).
• Binary, requiring the prefix 0b or 0B.
• Hexadecimal, requiring the prefix 0x or 0X.

The leading 0 for octal numbers can be thought of as the "O" in "Octal". The other prefixes use a leading zero to mark the beginning of a number that should not be interpreted as decimal. "B" is intuitively for "binary", while "X" is for "hexadecimal".

Answer 3

0b (or 0B) denotes a binary literal. C++ has allowed it since C++14. (It's not part of the C standard yet although some compilers allow it as an extension.) 0x (or 0X) is for hexadecimal.

0 can be used to denote an octal literal. (Interestingly 0 itself is an octal literal). Furthermore you use the escape sequence \ followed by digits to be read in octal: this applies only when defining const char[] literals using "" or char or multicharacter literals using ''. The '\0' notation that you often see to denote NUL when working with strings exploits that.

In the absence of a user defined literal suffix, any numeric literal starting with a non-zero is in denary.

There are rumblings in the C++ world to use 0o for an octal literal and perhaps even drop support for the leading zero version. Although that would be an hideous breaking change.