What does `::` do in C++ language?

Question

I found a C++ file in PARSEC benchmark suite and saw some functions like this:

long Rng::rand()
{
    return _rng->randInt();
}

what does the :: in the name of the function do here?

This does indeed look like a C++ method definition rather than a C function. Are you sure it was C code and not C++? — yzt, Jun 01 '13 at 07:13
I'm sorely tempted to rollback the edits by [devnull](http://stackoverflow.com/users/2235132/devnull) and [Randy Howard](http://stackoverflow.com/users/2159730/randy-howard) since they completely rewrite the question. The original question was about `::` in C; it has been revised so that C is no longer a tag or mentioned in the question, which makes my answer immaterial (because it addresses the original, C question and not the revised, C++ question). Edits are fine when they keep the intent of the original question. They aren't when they don't. — Jonathan Leffler, Jun 01 '13 at 07:56

score 10 · Answer 1 · edited Jun 01 '13 at 07:27

10

In C++ :: is the Scope resolution operator.
In this case it tells the compiler that it is a defintiion for rand() method which is a member function for Rng class/structure/union/namespace.

edited Jun 01 '13 at 07:27

Jonathan Leffler

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answered Jun 01 '13 at 07:07

Alok Save

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Given that `C` does not have `class`es, can you clarify your statement beginning with "In this case..."? – Shredderroy Jun 01 '13 at 07:09
3

@Shredderroy: The Q is tagged C++. – Alok Save Jun 01 '13 at 07:10
+1, but `Rng` could also be a namespace. – juanchopanza Jun 01 '13 at 07:16
@juanchopanza: True that. – Alok Save Jun 01 '13 at 07:22
Note that although this is the name for `::`, it's not an operator like the others. It doesn't operate on subexpressions. It joins *identifiers* to form *qualified-ids*, which are just more-specific names for things. So you could say it's "just" a punctuator, more like `;` than `+`. – Potatoswatter Jun 02 '13 at 14:50

score 10 · Answer 2 · answered Jun 01 '13 at 07:13

10

In C, :: is a syntax error unless it occurs inside a comment, a character literal or a string literal.

The :: can only appear in C++ code.

answered Jun 01 '13 at 07:13

Jonathan Leffler

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How can `::` appear inside a *character* literal? Am I missing something? – yzt Jun 01 '13 at 07:14
@H2CO3 And that isn't a syntax error? Two characters inside single quotes? – yzt Jun 01 '13 at 07:16
2

@yzt [nope](http://stackoverflow.com/questions/3960954/c-multicharacter-literal) – Jun 01 '13 at 07:16
1

@H2CO3 Hmmm... thanks! Something new everyday! – yzt Jun 01 '13 at 07:19
1

Multi-character character constants have an implementation-defined value, so they are not portable, but they are legitimate. See [Does anyone know of a C compiler that fails to compile this?](http://stackoverflow.com/questions/328215/does-anyone-know-of-a-c-compiler-that-fails-to-compile-this) for an extreme example. – Jonathan Leffler Jun 01 '13 at 07:20
Note that character literals/constants in C are actually ints - hence why older functions like getc, fgetc etc return ints, and functions like toupper and putc take ints instead of chars... Win32 programming makes heavy use of it with people giving things like timers and events integer values formed from four-character constants: `mainWnd->StartTimer('BEEP');`. – kfsone Jun 01 '13 at 08:11

What does `::` do in C++ language?

2 Answers2