What is the meaning of prepended double colon

Question

I found this line of a code in a class which I have to modify     Configuration   tmpCo   m configurationDB   pointer to current db   and I don t know what exactly means the double colon prepended to the class name  Without that I would read  declaration of tmpCo as a pointer to an object of the class Configuration    but the prepended double colon confuses me   I also found   typedef   config  set ConfigSet

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is a operator of defining the namespace   For example  if you want to use cout without mentioning using namespace std  in your code you write this   std  cout  lt  lt   test     When no namespace is mentioned  that it is said that class belongs to global namespace

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This answer is mostly for googlers  because OP has solved his problem already   The meaning of prepended    - scope resulution operator - has been described in other answers  but I d like to add why people are using it   The meaning is  take name from global namespace  not anything else   But why would this need to be spelled explicitly   Use case - namespace clash  When you have the same name in global namespace and in local nested namespace  the local one will be used  So if you want the global one  prepend it with     This case was described in  Wyatt Anderson s answer  plese see his example   Use case - emphasise non-member function  When you are writing a member function  a method   calls to other member function and calls to non-member  free  functions look alike   class A      void DoSomething           m counter 0                  Twist data                    Bend data                   if m counter gt 0  exit 0           int m couner             But it might happen that Twist is a sister member function of class A  and Bend is a free function  That is  Twist can use and modify m couner and Bend cannot  So if you want to ensure that m counter remains 0  you have to check Twist  but you don t need to check Bend   So to make this stand out more clearly  one can either write this- gt Twist to show the reader that Twist is a member function or write   Bend to show that Bend is free  Or both  This is very useful when you are doing or planning a refactoring

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its called scope resolution operator  A hidden global name can be referred to using the scope resolution operator    For example   int x  void f2        int x   1     hide global x      x   2     assign to global x    x   2     assign to local x

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is the scope resolution operator  It s used to specify the scope of something   For example     alone is the global scope  outside all other namespaces   some  thing can be interpreted in any of the following ways    some is a namespace  in the global scope  or an outer scope than the current one  and thing is a type  a function  an object or a nested namespace  some is a class available in the current scope and thing is a member object  function or type of the some class  in a class member function  some can be a base type of the current type  or the current type itself  and thing is then one member of this class  a type  function or object    You can also have nested scope  as in some  thing  bad  Here each name could be a type  an object or a namespace  In addition  the last one  bad  could also be a function  The others could not  since functions can t expose anything within their internal scope   So  back to your example    thing can be only something in the global scope  a type  a function  an object or a namespace    The way you use it suggests  used in a pointer declaration  that it s a type in the global scope    I hope this answer is complete and correct enough to help you understand scope resolution

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is used to link something   a variable  a function  a class  a typedef etc     to a namespace  or to a class   if there is no left hand side before     then it underlines the fact you are using the global namespace   e g      doMyGlobalFunction

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The    operator is called the scope-resolution operator and does just that  it resolves scope   So  by prefixing a type-name with this  it tells your compiler to look in the global namespace for the type   Example   int count   0   int main void      int count   0      count   1      set global count to 1   count   2        set local count to 2   return 0

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This ensures that resolution occurs from the global namespace  instead of starting at the namespace you re currently in  For instance  if you had two different classes called Configuration as such   class Configuration     class 1  in global namespace namespace MyApp       class Configuration     class 2  different from class 1     function blah                    resolves to MyApp  Configuration  class 2         Configuration  doStuff                  resolves to top-level Configuration  class 1           Configuration  doStuff                Basically  it allows you to traverse up to the global namespace since your name might get clobbered by a new definition inside another namespace  in this case MyApp

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represents scope resolution operator   Functions methods which have same name can be defined in two different classes  To access the methods of a particular class scope resolution operator is used

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Lots of reasonable answers already   I ll chip in with an analogy that may help some readers      works a lot like the filesystem directory separator      when searching your path for a program you d like to run   Consider    path to executable   This is very explicit - only an executable at that exact location in the filesystem tree can match this specification  irrespective of the PATH in effect   Similarly       std  cout      is equally explicit in the C   namespace  tree    Contrasting with such absolute paths  you can configure good UNIX shells  e g  zsh  to resolve relative paths under your current directory or any element in your PATH environment variable  so if PATH  usr bin  usr local bin  and you were  in   tmp  then     X11 xterm      would happily run  tmp X11 xterm if found  else  usr bin X11 xterm  else  usr local bin X11 xterm   Similarly  say you were in a namespace called X  and had a  using namespace Y  in effect  then     std  cout      could be found in any of   X  std  cout    std  cout    Y  std  cout  and possibly other places due to argument-dependent lookup  ADL  aka Koenig lookup    So  only   std  cout is really explicit about exactly which object you mean  but luckily nobody in their right mind would ever create their own class struct or namespace called  std   nor anything called  cout   so in practice using only std  cout is fine   Noteworthy differences   1  shells tend to use the first match using the ordering in PATH  whereas C   gives a compiler error when you ve been ambiguous   2  In C    names without any leading scope can be matched in the current namespace  while most UNIX shells only do that if you put   in the PATH   3  C   always searches the global namespace  like having   implicitly your PATH    General discussion on namespaces and explicitness of symbols  Using absolute   abc  def       paths  can sometimes be useful to isolate you from any other namespaces you re using  part of but don t really have control over the content of  or even other libraries that your library s client code also uses   On the other hand  it also couples you more tightly to the existing  absolute  location of the symbol  and you miss the advantages of implicit matching in namespaces  less coupling  easier mobility of code between namespaces  and more concise  readable source code   As with many things  it s a balancing act   The C   Standard puts lots of identifiers under std   that are less  unique  than cout  that programmers might use for something completely different in their code  e g  merge  includes  fill  generate  exchange  queue  toupper  max    Two unrelated non-Standard libraries have a far higher chance of using the same identifiers as the authors are generally un- or less-aware of each other   And libraries - including the C   Standard library - change their symbols over time   All this potentially creates ambiguity when recompiling old code  particularly when there s been heavy use of using namespaces  the worst thing you can do in this space is allow using namespaces in headers to escape the headers  scopes  such that an arbitrarily large amount of direct and indirect client code is unable to make their own decisions about which namespaces to use and how to manage ambiguities   So  a leading    is one tool in the C   programmer s toolbox to actively disambiguate a known clash  and or eliminate the possibility of future ambiguity

[c++] What is the meaning of prepended double colon "::"?

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