它依赖于编译器。也就是说，通常使用“将当前工作目录中的头文件优先于系统头文件”。<>通常用于系统头文件。从到规范(第6.10.2节):

A preprocessing directive of the form # include <h-char-sequence> new-line searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined. A preprocessing directive of the form # include "q-char-sequence" new-line causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read # include <h-char-sequence> new-line with the identical contained sequence (including > characters, if any) from the original directive.

因此，在大多数编译器中，使用“”首先检查您的本地目录，如果没有找到匹配，则继续检查系统路径。使用<>开始使用系统头文件进行搜索。

使用尖括号时，编译器会在包含路径列表中搜索文件。当你使用双引号时，它首先搜索当前目录(即正在编译的模块所在的目录)，然后才搜索包含路径列表。

因此，按照惯例，标准include使用尖括号，其他所有内容使用双引号。这可以确保在(不推荐)有与标准标头名称相同的本地标头的情况下，在每种情况下都会选择正确的标头。