Confusion on Unicode and Multibyte Articles_问答_开发者

By referring Joel's Article

Some people are under the misconception that Unicode is simply a 16-bit code where each character takes 16 bits and therefore there are 65,536 possible characters. This is not, actually, correct.

After reading the whole article, my point is that, if someone told you, his text is in unicode, you will have no idea how much memory space taken up 开发者_StackOverflowby every of his character. He have to tell you, "My unicode text is encoded in UTF-8", then only you will have idea how much memory space is taken up by every of his character.

Unicode = not necessary 2 byte for each character

However, when comes to Code Project's Article and Microsoft's Help, this confused me :

Microsoft :

Unicode is a 16-bit character encoding, providing enough encodings for all languages. All ASCII characters are included in Unicode as "widened" characters.

Code Project :

The Unicode character set is a "wide character" (2 bytes per character) set that contains every character available in every language, including all technical symbols and special publishing characters. Multibyte character set (MBCS) uses either 1 or 2 bytes per character

Unicode = 2 byte for each character ?

Is 65536 possible characters able to represent all language in this world?

Why the concept seems different among web developer community and desktop developer community?

Once upon a time,

Unicode had only as many characters as fit in 16 bits, and
UTF-8 did not exist or was not the de facto encoding to use.

These factors led to UTF-16 (or rather, what is now called UCS-2) to be considered synonymous with “Unicode”, because it was after all the encoding which supported all of Unicode.

Practically, you will see “Unicode” being used where “UTF-16” or “UCS-2” is meant. This is a historical confusion and should be ignored and not propagated. Unicode is a set of characters; UTF-8, UTF-16, and UCS-2 are different encodings.

(The difference between UTF-16 and UCS-2 is that UCS-2 is a true 16-bits-per-“character” encoding, and therefore encodes only the “BMP” (Basic Multilingual Plane) portion of Unicode, whereas UTF-16 uses “surrogate pairs” (for a total of 32 bits) to encode above-BMP characters.)

To expand on @Kevin's answer:

The description is Microsoft's Help is quite out of date, describing the state of the world in the NT 3.5/4.0 timeline.

You'll also occasionally see UTF-32 and UCS-4 mentioned as well, most often in the *nix world. UTF-32 is a 32-bit encoding of Unicode, a subset of UCS-4. The Unicode Standard Annex #19 describes the differences between them.

The best reference I've found describing the various encoding models is the Unicode Technical Report #17 Unicode Character Encoding Model, especially the tables in section 4.

Is 65536 possible characters able to represent all language in this world?

No.

Why the concept seems different among web developer community and desktop developer community?

Because Windows documentation is wrong. It took me a while to figure this out. MSDN says in at least two places that Unicode is a 16-bit encoding:

http://www.microsoft.com/typography/unicode/cscp.htm
http://msdn.microsoft.com/en-us/library/cwe8bzh0.aspx

One reason for the confusion is that at one point Unicode was a 16-bit encoding. From Wikipedia:

“Originally, both Unicode and ISO 10646 standards were meant to be fixed-width, with Unicode being 16 bit”

The other problem is that today in Windows APIs strings containing utf-16 encoded string data is usually represented using an array of wide characters, each one being 16-bits long. Despite that that Windows APIs support surrogate pairs of two 16-bit character types, to represent one Unicode code point.

Check out this blog post for more detailed information on the source of the confusion.