Global scope vs global namespace

Question

I saw usages of these two phrases: global scope and global namespace. What is the difference between them?

Answer 1

In C++, every name has its scope outside which it doesn't exist. A scope can be defined by many ways : it can be defined by namespace, functions, classes and just { }.

So a namespace, global or otherwise, defines a scope. The global namespace refers to using ::, and the symbols defined in this namespace are said to have global scope. A symbol, by default, exists in a global namespace, unless it is defined inside a block starts with keyword namespace, or it is a member of a class, or a local variable of a function:

int a; //this a is defined in global namespace        //which means, its scope is global. It exists everywhere.  namespace N {      int a;  //it is defined in a non-global namespace called `N`              //outside N it doesn't exist. } void f() {    int a;  //its scope is the function itself.            //outside the function, a doesn't exist.    {         int a; //the curly braces defines this a's scope!    } } class A {    int a;  //its scope is the class itself.            //outside A, it doesn't exist. }; 

Also note that a name can be hidden by inner scope defined by either namespace, function, or class. So the name a inside namespace N hides the name a in the global namspace. In the same way, the name in the function and class hides the name in the global namespace. If you face such situation, then you can use ::a to refer to the name defined in the global namespace:

int a = 10;  namespace N {     int a = 100;      void f()     {          int a = 1000;          std::cout << a << std::endl;      //prints 1000          std::cout << N::a << std::endl;   //prints 100           std::cout << ::a << std::endl;    //prints 10     } }