To some extent, the use of fully qualified names is the recommended way of referring to namespace members because it uniquely identifies and yet it avoids name conflicts. For example:

   #include   int main()  {    std::string str;    std::string str2;  }

In the example above, the fully qualified name std::string is used twice. In real world code, however, dozens of components of the Standard Library are used. Repeating their qualified names over and over again is laborious and error-prone. Worse yet, it renders your code unreadable. When you need to use qualified names more than 2-3 times, prefer a using-declaration or even a user-directive:

   #include   #include   int main()  {    using std::string; //using declaration; 
needed only once     using std::vector; // using declaration     string str; // non-qualified name     string str2;    vector  vi;  }