问题
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?
回答1:
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.
回答2:
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.
回答3:
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.
回答4:
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()
来源：https://stackoverflow.com/questions/7743356/length-of-a-c-stdstring-in-bytes