What does :: mean after declaring an object?

Question

I am trying to do some hello world stuff after completing the standard c++ tutorial. The first thing I tried doing is drawing directly on the screen without a window. I found this and it works.

#include "stdafx.h"
#include <Windows.h>
#include <iostream>
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    HDC screenDC = ::GetDC(0);
    ::Rectangle(screenDC, 200, 200, 300, 300);

    int exit; cin >> exit;
    return 0;
}

enter image description here

But nowhere in the standard c++ tutorial does it cover anything like this

HDC screenDC = ::GetDC(0);
::Rectangle(screenDC, 800, 200, 300, 300);

What is Rectangle a member of?

If you don't mind I have some other questions that might be simple. If not don't worry about it.

As soon as my rectangle has a window move over it, it disappears. Is there a callback to let me know about this so I can repaint?
How do I change the color of my rectangle?
Where can I go to learn about this? Every tutorial I have found has been sparse and most have been out of date. I can get a book if that's what I need to do. Have tried the following (sometimes with success and sometimes not):
http://msdn.microsoft.com/en-us/library/vstudio/bb384843.aspx
http://www.winprog.org/tutorial/
http://msdn.microsoft.com/en-us/library/ms533895(v=vs.85).aspx

That is the *scope resolution operator*. In this case, `::Rectangle` means that the function `Rectangle` lives in the global namespace — Andy Prowl, May 14 '13 at 17:21
possible duplicate of [What is the meaning of prepended double colon "::" to class name?](http://stackoverflow.com/questions/4269034/what-is-the-meaning-of-prepended-double-colon-to-class-name) — Moo-Juice, May 14 '13 at 17:24
It's a much better idea to have a topmost, fullscreen, colour key window instead of drawing on the screen. Use `CreateSolidBrush` and select it into the DC or something to change colour (there's a lot of info on GDI on MSDN), and Petzold's book is wonderful. — chris, May 14 '13 at 17:27
Please don't add additional questions not relevant to the original question - create new questions for these items. — Moo-Juice, May 14 '13 at 17:27

score 9 · Answer 1 · answered May 14 '13 at 17:23

9

:: is the scope resolution operator.

scope::name means use the name declared in the class or namespace called scope.

::name means use the name declared in the global namespace. Usually, it's optional; but sometimes you need it if there's something else with the same name in the current scope.

answered May 14 '13 at 17:23

Mike Seymour

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Specifically, `::` is the [scope resolution operator](http://publib.boulder.ibm.com/infocenter/comphelp/v7v91/index.jsp?topic=%2Fcom.ibm.vacpp7a.doc%2Flanguage%2Fref%2Fclrc05cplr175.htm). – Caleb May 14 '13 at 17:25
@Caleb: Yeah, I guess I should mention that. I assumed the OP knew what it was (since he asked "What is Rectangle a member of?"), but completeness is good. – Mike Seymour May 14 '13 at 17:27

RandyGaul · Accepted Answer · 2013-05-14T17:35:55.973

7

:: like that accesses the global namespace. It is namespace resolution. You can just drop those :: if you like, they are extra cruff.

edited May 14 '13 at 17:35

answered May 14 '13 at 17:22

RandyGaul

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You can drop them here; but sometimes you need them to resolve ambiguities. – Mike Seymour May 14 '13 at 17:25
Well, you can drop them in *this case*. But if you're currently in a different namespace, which also has something with the same name, just writing the symbol name will be ambiguous. – leemes May 14 '13 at 17:25

score 2 · Answer 3 · answered May 14 '13 at 17:25

:: is the scope resolution operator. ::Identifier will refer to anything in the global namespace. It is not always necessary however. It's only really needed when not using it will cause an ambiguity:

int i = 0;

void func() {
    int i = 0;

    ::i = 1; // This will set the global i
    i = 1; // This will set the local i
}

(Note: I do not endorse globals in any way)

score 1 · Answer 4 · answered May 14 '13 at 17:25

:: is used to access things that are in classes or namespaces. (Or in this case, that are not.)

So if you have a class Foo with a static method bar you can call it like this Foo::Bar();

Also, if you have a namespace MyLibrary with a function PrintPrettyThings you can call it like this: MyLibrary::PrintPrettyThings();

And if you have another function Bar somewhere and you are inside a method of Foo you can use ::Bar(); to call the Bar that is outside of Foo, otherwise you will just call Foo::Bar.

What does :: mean after declaring an object?

4 Answers4