What Does "0b" And "0x" Stand For When Assigning Binary And Hex?

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Learn more about Labs What does "0b" and "0x" stand for when assigning binary and hex? Ask Question Asked 5 years, 3 months ago Modified 3 years, 9 months ago Viewed 50k times 23

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?

• c++
• binary
• hex

Share Improve this question Follow edited Feb 22, 2021 at 13:49 Neuron 5,75355 gold badges4343 silver badges6262 bronze badges asked Aug 22, 2019 at 8:34 rrswarrswa 1,05011 gold badge99 silver badges2525 bronze badges 5

• "Is there one for octal as an example?" Sure that's just a 0 prefix. – πάντα ῥεῖ Commented Aug 22, 2019 at 8:37
• 1 Intuitively, "b" is for binary and "x" for hex. – Sweeper Commented Aug 22, 2019 at 8:38
• these numbers are called "literals" and if you google you'll see all the literal syntax prefixes/suffixes. – Cory Nelson Commented Aug 22, 2019 at 8:38
• 1 en.cppreference.com/w/cpp/language/integer_literal – πάντα ῥεῖ Commented Aug 22, 2019 at 8:43
• 8 "x" is "exadecimal", because programming languages used to be manufactured exclusively in Manchester. – molbdnilo Commented Aug 22, 2019 at 8:46

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3 Answers 3

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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".

Share Improve this answer Follow answered Aug 22, 2019 at 8:50 StoryTeller - Unslander MonicaStoryTeller - Unslander Monica 170k2222 gold badges393393 silver badges472472 bronze badges 4

• 1 Given you've taken the "standard" route, you might want to mention things like \123. – Bathsheba Commented Aug 22, 2019 at 8:56
• 3 @Bathsheba - '\123' is not strictly valid. One can only have octal and hex escape sequence. Also, since the issue at hand is integer literals, I feel personally that bringing character literals into scope can be confusing. Two monstrous grammar productions in a single answer feels like too much to me. – StoryTeller - Unslander Monica Commented Aug 22, 2019 at 9:03
• Why 0x chosen, instead of 0h? And 0o is better than 0, isn't it? – DawnSong Commented Sep 6, 2021 at 14:39
• 2 @DawnSong - Phonetically, the X stands out more than the H. Plus, the o seems redundant. In certain jargon, o and 0 are used interchangeably. 08:00 might be referred to as "oh eighthundred hours" for instance. – StoryTeller - Unslander Monica Commented Sep 6, 2021 at 14:45

Add a comment | 4

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.

Share Improve this answer Follow edited Aug 22, 2019 at 9:05 answered Aug 22, 2019 at 8:40 BathshebaBathsheba 234k3535 gold badges369369 silver badges496496 bronze badges 2

• @Angew: Indeed, I've spelt it out. You can use them in funnies such as multicharacter constants too, although that one could be for the pub quiz. – Bathsheba Commented Aug 22, 2019 at 9:03
• In short, they're a type of character escape sequence, and not an integer literal. – Angew is no longer proud of SO Commented Aug 22, 2019 at 9:04

Add a comment | 2

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.

Share Improve this answer Follow edited Aug 22, 2019 at 8:50 answered Aug 22, 2019 at 8:45 eerorikaeerorika 238k1212 gold badges206206 silver badges345345 bronze badges Add a comment |

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