Is there any example that triple colon (:::) is a valid syntax in c++ code?

Question

In my impression it seems I saw something like that:

A:::b()

But I don't remember if it is c++,also after searched "triple colon" in c++ but seems rare information about it in c++, is there any example that the code can have 3 colons(:::) in the code which is valid syntax?

Answer 1

This is slightly off topic because it refers to inline assembly rather than C++ per se. However, here's a tip for those who got here (as I did) after seeing this operator in the following expression:

asm volatile("" ::: "memory");

This syntax is used by gcc to tell the compiler to create a 'memory barrier'. Wikipedia's Memory Ordering topic is a better explanation than I can provide.

Answer 2

Given a global function b:

int b() {
    return 5;    
}

There are two (actually a couple more per Steephen's answer if you get creative¹) cases where this would work, but only with a space:

1. As a label:

   int main() {
       A: ::b();
   }
   

2. As part of a conditional expression:

   int main() {
       int A = 0;
       int result = true ? A: ::b();
   }
   

The reason the space is needed is because C++ lexing is greedy and always lexes ::: as :: followed by :, never : followed by ::. (A notable exception to this is >> being allowed to end two template parameter/argument lists only since a special change C++11.)

---

1: See examples of access modifiers, inheritance, constructor initializer list.

Answer 3

Yes, there are valid C++ programs which include the sequence of characters ::: (outside of comments and quoted literals).

Normally, that sequence would be lexically analysed as a :: token (the scope resolution operator) followed by a : token. As far as I know, there is no valid C program of this form, because a : can only be preceded by a name (as a label) or an expression (either in a case label or as part of the ternary ?: operator). And :: cannot be the last token in an expression.

You can force the lexical analyser to produce a : followed by a :: but only by placing whitespace between the two tokens, making : ::. And, of course, you can use the preprocessor to define a macro which ignores or stringifies an argument, so ::: could appear as an argument to such a macro.

Leaving those aside, as far as I know the only possibility is when the first : is the second character in a <: token. For example:

const int size = 42;
int A<:::size:>;

(Live on coliru.)

If that looks weird, remember that <: is an alternative spelling for [ and :> is an alternative spelling for ]. The unary scope resolution operator :: indicates that the name it qualifies is in global scope.

Curiously, <:: is an exception to the maximal-munch rule if it is not followed by : or >, in which case it must be analysed as < followed by :: rather than as <: followed by :. However, when it is followed by : (as above) or >, maximal munch still applies and the first token is <:.

Answer 4

No, there is no triple colon(:::) as operator in C++

There is scope resolution operator, it is double colon(::).

There is single colon(:) that you can see along with access specifiers like private, public and protected. And also within constructor syntax