Question

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What is the difference between #include <filename> and #include “filename”?

When I written my programs, I include libraries like #include <iostream> . but also #include "iostream" is correct and the code would compiled without any error or problem.

So, I’m wondering what’s the difference between < > and " " ?

Answer 1

When you use < >, the compiler only looks in the system-designated directory/directories (e.g., whatever you’ve set in the include environment variable) for the header.

When you use " ", the compiler looks in the local directory first, and if that fails, re-searches just like you’d used < >. Technically, (i.e., according to the standard) that doesn’t have to be the “local” directory, but that’s how it works in essentially every compiler of which I’m aware).

In case you care, the official wording from the standard is actually kind of vague (§16.2/2-3):

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.