How is this bracketless/braceless code valid? [duplicate]

Answer 1

Digraph, introduced in C99.

C99 6.4.6 Punctuators

In all aspects of the language, the six tokens

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

behave, respectively, the same as the six tokens

[ ]  {} # ##

except for their spelling.

Answer 2

This code uses C digraphs, as explained here: http://en.wikipedia.org/wiki/Digraphs_and_trigraphs#C

The C standard (ISO/IEC 9899:201x, Punctuators §6.4.6:3) has this to say on the matter:

In all aspects of the language, the six tokens

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

behave, respectively, the same as the six tokens

[ ] { } # ##

except for their spelling.

Answer 3

These are punctuators and %: is the same as # and %> is the same as } etc... The C99 draft standard section 6.4.6 Punctuators says:

In all aspects of the language, the six tokens67)

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

behave, respectively, the same as the six tokens

  [   ] {    }   #   ##

They are also sometimes refereed to as digraphs as we see in note 67:

These tokens are sometimes called ‘‘digraphs’’.

Answer 4

It is all a very Zen little corner of the C language.

In the olden days, terminals and keyboards often didn't support all the characters one might want for programming; so digraphs and trigraphs were introduced.

I'm surprised any modern compiler still supports them, but there it is.