Difference Between Angle Bracket ≪ ≫ and Double Quotes " " While Including Header Files in C++

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

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

C - Should I use quotes or brackets to include headers in a separate directory

The common convention is:

• Use < … > for headers that are part of the C implementation or the platform—headers outside your project such as the C standard library, Unix or Windows headers, and headers of libraries generally installed for your development environment.
• Use " … " for headers that are part of your project.

This is not fully determined by the C standard; it is a matter of general practice. For each delimiter choice, a compiler has a list of places (a search path) where it looks for headers. Those search paths are commonly designed to facilitate the use described above, but they are customizable (depending on the compiler you use) by command-line switches, by environment variables, by system settings, and/or by settings made when building the compiler.

Here is what the C standard says about them in C 2018 6.10.2. Paragraph 2 says:

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.

Paragraph 3 says:

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.

Note some of the differences between the two:

• The text for the bracket form says it searches for a header identified uniquely. The text for the quote form does not include the word “uniquely”. This suggests all the headers referred to by the bracketed form are supposed to be different from each other, which you might expect if they were part of a designed system seeking to avoid ambiguity.
• Note that it says the first form “searches a sequence of implementation-defined places.” This accords with the compiler having a list of places to search for standard headers. For the second form, it uses “the source file identified by the specified sequence.” This accords with using the text between quotes as a path in the file system.

This text in the standard is quite lax, both allowing implementation-defined methods of identifying the files, so either can be stretched to be the same as the other (although it would be interesting to see a compiler complain that a header named in brackets is not unique), and compiler configuration options are sufficiently broad that you could use each in either way for your project. However, it is generally better to stick to convention.

#import using angle brackets < > and quote marks

Objective-C has this in common with C/C++; the quoted form is for "local" includes of files (you need to specify the relative path from the current file, e.g. #include "headers/my_header.h"), while the angle-bracket form is for "global" includes -- those found somewhere on the include path passed to the compiler (e.g. #include <math.h>).

So to have your own headers use < > not " " you need to pass either the relative or the absolute path for your header directory to the compiler. See "How to add a global include path for Xcode" for info on how to do that in Xcode.

See this MSDN page for more info.

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