What is the difference between ::std::string and std::string?

Question

what is the difference between ::std::string and std::string The former is global? But global for what?Is not namespace std global? Thanks for helping me.

Answer 1

::std::string means string in namespace std in the global namespace. The leading :: forces the lookup to start in the global namespace. Therefore ::std::string always means the string type from the C++ standard library.

std::string means string in namespace std, where std will be looked up in the current scope. Therefore, if there is a class, namespace, or enum named std, name lookup might find that std.

#include <string>
namespace foo {
  namespace std {
    class string { ... };
    namespace bar {
      std::string s;    // means foo::std::string
      ::std::string s;  // means string from standard library
    }
  }
}

It is not necessary to use the leading :: as long as you and your collaborators agree to not name anything std. It's just good style.