Using in C

Question

I am learning C   and I can never tell when I need to use      I do know that I need to use std   in front of cout and cin  Does this mean that inside of the iostream file the developers that created it  made a namespace called std and put the functions cin and cout into the namespace called std  When I created a new class that isn t in the same file as main   for some reason I must add       For example  if I create a class called A   why do I need to put A   in front of a function that I make  even though I didn t put it into a namesace  For example void A  printStuff       If I create a function in main  why don t I have to put main  printStuf     I know that my question is probably confusing  but could someone help me

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The    is called scope resolution operator   Can be used like this      identifier class-name    identifier namespace    identifier  You can read about it here https   docs microsoft com en-us cpp cpp scope-resolution-operator view vs-2017

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look at it is informative  Qualified identifiers  A qualified id-expression is an unqualified id-expression prepended by a scope resolution operator     and optionally  a sequence of enumeration   since C  11 class or namespace names or decltype expressions  since C  11  separated by scope resolution operators  For example  the expression std  string  npos is an expression that names the static member npos in the class string in namespace std  The expression   tolower names the function tolower in the global namespace  The expression   std  cout names the global variable cout in namespace std  which is a top-level namespace  The expression boost  signals2  connection names the type connection declared in namespace signals2  which is declared in namespace boost   The keyword template may appear in qualified identifiers as necessary to disambiguate dependent template names 1

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The    are used to dereference scopes   const int x   5   namespace foo     const int x   0     int bar       int x   1    return x     struct Meh     static const int x   2     int main       std  cout  lt  lt  x       gt  5         int x   4      std  cout  lt  lt  x       gt  4     std  cout  lt  lt    x       gt  5  this one looks for x outside the current scope       std  cout  lt  lt  Meh  x       gt  2  use the definition of x inside the scope of Meh   std  cout  lt  lt  foo  x       gt  0  use the definition of x inside foo   std  cout  lt  lt  bar         gt  1  use the definition of x inside bar  returned by bar      unrelated  cout and cin are not functions  but instances of stream objects   EDIT fixed as Keine Lust suggested

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One use for the  Unary Scope Resolution Operator  or  Colon Colon Operator  is for local and global variable selection of identical names       include  lt iostream gt      using namespace std           int variable   20           int main             float variable   30           cout  lt  lt   quot This is local to the main function   quot   lt  lt  variable  lt  lt  endl      cout  lt  lt   quot This is global to the main function   quot   lt  lt    variable  lt  lt  endl           return 0         The resulting output would be   This is local to the main function  30 This is global to the main function  20  Other uses could be  Defining a function from outside of a class  to access a static variable within a class or to use multiple inheritance

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You re pretty much right about cout and cin  They are objects  not functions  defined inside the std namespace  Here are their declarations as defined by the C   standard   Header  lt iostream gt  synopsis  include  lt ios gt   include  lt streambuf gt   include  lt istream gt   include  lt ostream gt   namespace std     extern istream cin    extern ostream cout    extern ostream cerr    extern ostream clog     extern wistream wcin    extern wostream wcout    extern wostream wcerr    extern wostream wclog         is known as the scope resolution operator  The names cout and cin are defined within std  so we have to qualify their names with std    Classes behave a little like namespaces in that the names declared inside the class belong to the class  For example  class foo     public      foo        void bar        The constructor named foo is a member of the class named foo  They have the same name because its the constructor  The function bar is also a member of foo  Because they are members of foo  when referring to them from outside the class  we have to qualify their names  After all  they belong to that class  So if you re going to define the constructor and bar outside the class  you need to do it like so  foo  foo          Implement the constructor    void foo  bar          Implement bar    This is because they are being defined outside the class  If you had not put the foo   qualification on the names  you would be defining some new functions in the global scope  rather than as members of foo  For example  this is entirely different bar  void bar          Implement different bar    It s allowed to have the same name as the function in the foo class because it s in a different scope  This bar is in the global scope  whereas the other bar belonged to the foo class

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