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What is ':-!!' in C?

There is no operator in C spelled :-!!. It’s not a defined token or sequence in the C language. If you see :-!! in code, it’s most likely a trick, a joke, or simply invalid syntax that won’t compile under a standard-compliant C compiler.

Potential Misinterpretations

  1. Emoticon or Typo
    :-!! can look like an emoticon. Some programmers use emoticon-like symbols in comments or as jokes in code, but it has no meaning as an operator.

  2. Label and Logical Negation
    In C, you might see something like:

    label: !!x;

    This is valid code, meaning:

    • label: is a goto label (which ends with a colon).
    • !!x is double logical negation, converting x to a boolean (0 or 1).

    However, writing them together without a space—like label:!!x;—would still be parsed as a label followed by a double negation expression, not a single operator. And :-!! does not form a valid label or operator on its own.

  3. Trigraph Confusion
    Certain sequences in older C standards (trigraphs, e.g., ??!, ??=, etc.) can transform into single characters, but :-!! is not one of them. No part of :-!! is a valid trigraph or digraph, so it won’t get replaced by the preprocessor.

Why It Doesn’t Exist

  • No Grammar Rule: The C grammar has rules for operators (e.g., +, -, ++, --, &&, ||, etc.), labels (which must end with a colon), and trigraph sequences. None of these rules allow for :-!! as a token or operator.
  • Won’t Compile: If you place :-!! plainly in your code (not as part of an emoticon in a comment or string), your compiler will complain about a syntax error or invalid token.

Example of a Possible “Look-Alike”

You could validly write:

#include <stdio.h>

int main(void) {
    int x = 5;

label: 
    printf("%d\n", !!x); // Double negation of x => 1

    return 0;
}

Here:

  • label: is a goto label.
  • !!x is double logical negation of x.
    But there is no :-!! operator—this is just a label followed by a separate expression.

Mastering C and Beyond

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These resources will help you hone your coding skills in C, C++, and other languages, and give you a structured approach to tackling complex problems—all while avoiding confusion around unusual or invalid syntax.

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