why STL header files have no extension?

问题

I got this basic doubt. The STL header doesn't have .h extension.

#include <vector>
#include <map>

Is there is any specific reason behind this? Anybody knows history behind this, please share.

EDIT:

@GMan found Michael Burr's answer which addresses this question.

回答1:

• The #include directive doesn't discriminate file types (it's just a glorified copy-paste operation) - no automatic adding of .h is happening.
• C++ standard header files are provided without the .h extension
• Sometimes backward compatibility header files are provided by the vendor with the same name with the .h extension added

It all has to do with namespaces. The .h counterparts for C++ standard headers usually #includes the proper C++ standard header (without .h extension) and then issues a bunch of using (something like this):

FILE: iostream.h

#include <iostream>

using std::iostream;
using std::ostream;
using std::ios;
...

whereas the headerfile without the .h extension does not pollute the namespace with all the defined classes and types.

来源：https://stackoverflow.com/questions/901216/why-stl-header-files-have-no-extension