What is use of name space std ??

Answer 1

well everything in standard C++ library is in this namespace. It makes moving through the classes easier (since you can call std:: and you see all functions/classes). Also it can save you many errors. For example if you'd like to have your own "cout" class it would collide with C++ standard cout. But since you have to call std:: first there will be no error.

Answer 2

thanks @jakub stasiak