The sequence of characters between < and > uniquely refer to a header, which isn't necessarily a file. Implementations are pretty much free to use the character sequence as they wish. (Mostly, however, just treat it as a file name and do a search in the include path, as the other posts state.)
If the
#include "file"form is used, the implementation first looks for a file of the given name, if supported. If not (supported), or if the search fails, the implementation behaves as though the other (#include <file>) form was used.Also, a third form exists and is used when the
#includedirective doesn't match either of the forms above. In this form, some basic preprocessing (such as macro expansion) is done on the "operands" of the#includedirective, and the result is expected to match one of the two other forms.--
The simple general rule is to Use angled brackets to include header files that come with the compiler. Use double quotes to include any other header files.
简单地说，系统头文件用尖括号，比如#include <iostream>，尖括号里面的不一定是个文件名
其他的（自己写的头文件，下载的包等）用双引号