What is the significance of the <: syntax in C? [duplicate]

Answer 1

It's a mostly historic thing to deal with terminal and code page limitations.

Read the short Wikipedia article on digraphs and trigraphs.

Answer 2

It's called digraph. It was used for when terminals didn't have some of the characters used by C.

Answer 3

Its call alternative tokens. C++ have several of them:

 <%     {
 %>     }
 <:     [
 :>     ]
 %:     #
 %:%:   ##
 and    &&
 bitor  |
 or     ||
 xor    ˆ
 compl  ~
 bitand &
 and_eq &=
 or_eq  |=
 xor_eq ˆ=
 not    !
 not_eq !=

You can seen some of the alternative token consists of letters. So you can write if (a<b and b<c) in a compiler which can correctly handle them. Their existence is for lack of symbols in keyboards or character sets. The alternative tokens are never replaced with the primary one (unlike trigraphs), but them behave the same as the primary one.

However, C++0x require special treatment for <:: (2.5p3):

Otherwise, if the next three characters are <:: and the subsequent character is neither : nor >, the < is treated as a preprocessor token by itself and not as the first character of the alternative token <:.

So that SomeTemplate<::SomeClass> can be correctly handled.

Answer 4

It's for charsets that don't have [.

§6.4.6-3 (C99)

In all aspects of the language, the six tokens

<: :> <% %> %: %:%:

behave, respectively, the same as the six tokens

[  ]  {  }  #  ##