3.7. NamespacesΒΆ

All functions in C++ are by default global. Another way of saying this is that they are by default in the global namespace. The namespace keyword provides a mechanism to avoid polluting the global namespace with too many names.

A namespace is simply a named block that defines a scope. Namespaces provide a method for preventing name conflicts in large projects.

Symbols declared inside a namespace block are placed in a named scope that prevents them from being mistaken for identically-named symbols in other scopes.

Multiple namespace blocks with the same name are allowed. All declarations within those blocks are declared in the named scope.

// declare some things in the mesa namespace
namespace mesa {
  int i = 0;
  double pi = 3.1416;
  void details (char);
}

void mesa::details (char c) { // define the function declared earlier
  std::cout << int{c};
}

//void mesa::oops () {       // error: oops not yet declared in mesa namespace
//}

namespace mesa {      // a separate mesa namespace block
  void oops ();       // a declaration with no definition is allowed
  namespace cisc {
    double pi = 3.141592653;  // not the same variable as mesa::pi
  }
}

int main () {
  using mesa::cisc::pi;  // we can specify a name to treat as if it was global
  pi = 3.f;
  mesa::details('a');
}

The larger your project, the more important it is to partition the global namespace. By default, all symbols are declared in the global namespace (::).

What is the problem with the global namespace?

Well-behaved third party libraries will not put much (if anything) in the global namespace.

You can put anything in a namespace, except main. The function main has a few special rules and one is that it must be in the global namespace.

The using directive allows all the names in a namespace to be used without the namespace-name as an explicit qualifier. Use a using directive in an implementation file (i.e. *.cpp) if you are using several different identifiers in a namespace; if you are just using one or two identifiers, then consider a using declaration to only bring those identifiers into scope and not all the identifiers in the namespace. If a local variable has the same name as a namespace variable, the namespace variable is hidden. It is an error to have a namespace variable with the same name as a global variable.

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