Length of a C++ std::string in bytes

Question

I'm having some trouble figuring out the exact semantics of std::string.length(). The documentation explicitly points out that length() returns the number of characters in the string and not the number of bytes. I was wondering in which cases this actually makes a difference.

In particular, is this only relevant to non-char instantiations of std::basic_string<> or can I also get into trouble when storing UTF-8 strings with multi-byte characters? Does the standard allow for length() to be UTF8-aware?

there is wstring for UTF and there it makes senses that length returns the number of characters since the character's size could vary. — AndersK, Oct 12 '11 at 16:33
@AndersK.: No, `wchar_t` has a fixed size like any other type. It can't magically vary. — Lightness Races in Orbit, Oct 12 '11 at 16:33
Also check this lovely thread about `std::string` vs. `std::wstring` and some stuff about Unicode: http://stackoverflow.com/questions/402283/stdwstring-vs-stdstring — wkl, Oct 12 '11 at 16:35
@AndersK.: `wstring` has nothing to do with UTF. Perhaps you were thinking of `u16string` or `u32string`? — Kerrek SB, Oct 12 '11 at 17:27

score 31 · Accepted Answer · answered Oct 12 '11 at 16:33

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When dealing with non-char instantiations of std::basic_string<>, sure, length may not equal number of bytes. This is particularly evident with std::wstring:

std::wstring ws = L"hi";
cout << ws.length();     // <-- 2, not 4

But std::string is about char characters; there is no such thing as a multi-byte character as far as std::string is concerned, whether you crammed one in at a high level or not. So, std::string.length() is always the number of bytes represented by the string. Note that if you're cramming multibyte "characters" into an std::string, then your definition of "character" suddenly becomes at odds with that of the container and of the standard.

answered Oct 12 '11 at 16:33

Lightness Races in Orbit

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That makes perfect sense. I simply got confused by the wording in the documentation here. Thanks for clearing things up. – ComicSansMS Oct 12 '11 at 16:40
@ComicSansMS: Not a problem :) – Lightness Races in Orbit Oct 12 '11 at 16:41
*But `std::string` is about `char` characters*, so the definition of "character" in C++ is "element of some string type", rather than "what a human sees, encoded" or "a unicode codepoint, encoded somehow". This sounds believable, but can anyone quote chapter-and-verse on this? – Adrian Ratnapala Jan 08 '12 at 10:14
@AdrianRatnapala: It's less that the standard says it doesn't care about encodings, and more about it not saying that it does. Still, `2.3/1` might be of interest - it defines the "basic character set". And `2.3/3` says: `The execution character set and the execution wide-character set are implementation-defined supersets of the basic execution character set and the basic execution wide-character set, respectively. The values of the members of the execution character sets and the sets of additional members are locale-specific.` – Lightness Races in Orbit Jan 08 '12 at 17:18
Well, I guess that's what I get for asking for chapter-and-verse. – Adrian Ratnapala Jan 08 '12 at 18:02
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@AdrianRatnapala: Yes, when asking for chapter-and-verse, you get chapter-and-verse. Anything else I can help you with? :) – Lightness Races in Orbit Jan 08 '12 at 18:24

score 12 · Answer 2 · edited May 23 '17 at 12:00

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If we are talking specifically about std::string, then length() does return the number of bytes.

This is because a std::string is a basic_string of chars, and the C++ Standard defines the size of one char to be exactly one byte.

Note that the Standard doesn't say how many bits are in a byte, but that's another story entirely and you probably don't care.

EDIT: The Standard does say that an implementation shall provide a definition for CHAR_BIT which says how many bits are in a byte.

By the way, if you go down a road where you do care how many bits are in a byte, you might consider reading this.

edited May 23 '17 at 12:00

Community

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answered Oct 12 '11 at 16:39

John Dibling

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Indeed, "byte" is not necessarily synonymous with "octet". – Lightness Races in Orbit Oct 12 '11 at 16:42
The standard does define `CHAR_BIT`, the number of bits in a byte. – Mike Seymour Oct 12 '11 at 17:01
@Mike: True, but the Standard doesn't say what that's defined to. When I said "doesn't say how many bits are in a byte" I meant in a precise, unambigious sense. But I'll clarify my post with an edit, thanks for pointing this out. – John Dibling Oct 12 '11 at 17:05

NuSkooler · Answer 3 · 2015-01-23T17:00:22.433

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A std::string is std::basic_string<char>, so s.length() * sizeof(char) = byte length. Also, std::string knows nothing of UTF-8, so you're going to get the byte size even if that's not really what you're after.

If you have UTF-8 data in a std::string, you'll need to use something else such as ICU to get the "real" length.

edited Jan 23 '15 at 17:00

answered Oct 12 '11 at 16:39

NuSkooler

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score 0 · Answer 4 · answered May 28 '13 at 16:17

cplusplus.com is not "the documentation" for std::string, it's a poor quality site full of poor quality information. The C++ standard defines it very clearly:

21.1 [strings.general] ¶1

This Clause describes components for manipulating sequences of any non-array POD (3.9) type. In this Clause such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.
21.4.4 [string.capacity] ¶1

size_type size() const noexcept;
Returns: A count of the number of char-like objects currently in the string.
Complexity: constant time.

size_type length() const noexcept;
Returns: size()

Length of a C++ std::string in bytes

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