Difference between angle bracket < > and double quotes " " while including header files in C++?

Answer 1

It's compiler dependent. That said, in general using " prioritizes headers in the current working directory over system headers. <> usually is used for system header. From to the spec (Section 6.10.2):

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.

So on most compilers, using the "" first checks your local directory, and if it doesn't find a match then moves on to check the system paths. Using <> starts the search with system headers.

Answer 2

When you use the angle brackets, the compiler searches for the file in the include path list. When you use the double quotes, it first searches the current directory (i.e. the directory where the module being compiled is) and only then it'll search the include path list.

So, by convention, you use the angle brackets for standard includes and the double quotes for everything else. This will assure that in the (not recommended) case in which you have a local header with the same name as a standard header, the right one will be chosen in each case.