I understand that if there is not using namespace std, and you want to write a cout, you need to have a std::cout. What does the std represents? Why is std widely used (std::vector,std::cout,std::cin)?

CodePudding user response：

It's a namespace reserved for the C standard library.

A good choice historically: any token starting with std is reserved in C. So that meant that early C compilers would not break existing C code. Of course the languages have diverged since then. Etymologically, std is an abbreviation for standard.

CodePudding user response：

std stands for "standard".

The reason why so much standard stuff goes in the std namespace is simple: Before namespaces, different code written by different people would often use the same name and cause a conflict. For example, my drink dispenser program from 1994 might have a class ofstream which is an orange fanta stream. When a new version of C came along and added ofstream which was an output file stream, my program wouldn't compile any more, or it'd crash.

Okay, orange-fanta-stream is silly, but major operating systems do have C functions called open, close, and index. I'm sure many people have tried to make global variables called open, and then their programs have crashed.

In C , all the new C standard stuff is inside std::, so as long I don't call something in my program std, they can add new stuff inside std:: and it definitely won't cause this problem. Unfortunately all the stuff that C inherits from C is outside std::, so you still can't make a global variable called open (on Linux), but at least it's a start.