c++包含目錄中"" 的區別

<>先去系統目錄中找頭文件，如果沒有在到當前目錄下找。所以像標準的頭文件 stdio.h、stdlib.h等用這個方法。 


而""首先在當前目錄下尋找，如果找不到，再到系統目錄中尋找。 這個用於include自定義的頭文件，讓系統優先使用當前目錄中定義的。 



When writing your C program, you can include files in two ways. The first way is to surround the file you want to include with the angled brackets < and >. This method of inclusion tells the preprocessor to look for the file in the predefined default location. 

This predefined default location is often an INCLUDE environment variable that denotes the path to your include files. For instance, given the INCLUDE variable 

INCLUDE=C:COMPILERINCLUDE;S:SOURCEHEADERS;

using the #include  version of file inclusion, the compiler first checks the C:COMPILERINCLUDE 
directory for the specified file. If the file is not found there, the compiler then checks the S:SOURCEHEADERS directory. If the file is still not found, the preprocessor checks the current directory. 

The second way to include files is to surround the file you want to include with double quotation marks. This method of inclusion tells the preprocessor to look for the file in the current directory first, then look for it in the predefined locations you have set up. 

Using the #include  version of file inclusion and applying it to the preceding example, the preprocessor first checks the current directory for the specified file. If the file is not found in the current directory, the C:COMPILERINCLUDE directory is searched. If the file is still not found, the preprocessor checks the S:SOURCEHEADERS directory. 

• The #include  method of file inclusion is often used to include standard headers such as stdio.h or stdlib.h. This is because these headers are rarely (if ever) modified, and they should always be read from your compiler&rsquo;s standard include file directory.
  

• The #include method of file inclusion is often used to include nonstandard header files that you have created for use in your program. This is because these headers are often modified in the current directory, and you will want the preprocessor to use your newly modified version of the header rather than the older, unmodified version.