c++ - What is the difference between #include <filename> and #include "filename"?

Question

In the C and C++ programming languages, what is the difference between using angle brackets and using quotes in an include statement, as follows?

1. #include <filename>
2. #include "filename"

Answer 1

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.