The order of include header files and the difference between double quotation marks "and angle brackets < >

Double quotes ”” And angle brackets <> The difference between

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• ​ ​Include The order of header files ​​
• ​ ​ The difference between double quotation marks and angle brackets ：​​
• ​ ​ Current header file directory ​​
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Include The order of header files

about include For the header file , If it's in the file a.h Declare a in the file b.h

Variables defined in , Without reference b.h. Then you have to be in a.c Reference in file b.h file , And first quote b.h, Post reference

a.h, Otherwise, the report variable type does not declare an error .

The difference between double quotation marks and angle brackets ：

The path to find the header file in the compiler preprocessing phase is different .

For header files contained in double quotation marks , The order to find the header file path is ：

Current header file directory

Header file path set by compiler （ The compiler can use -I Explicitly specify the search path ）

System variables CPLUS_INCLUDE_PATH/C_INCLUDE_PATH Specified header file path

For header files included with angle brackets , The order of searching the header file is ：

Header file path set by compiler （ The compiler can use -I Explicitly specify the search path ）

System variables CPLUS_INCLUDE_PATH/C_INCLUDE_PATH Specified header file path