#include "file" -vs- #include <file>

[Victor Bazarov]

I'm slightly confused about when to use parens around #included files

You're not alone.

and when to use angle brackets. I understand (I think) that the
difference is that the compiler will search in its standard include
directories for files included as <file> but not for those included as
"file". It's clear to me then that headers for the standard libraries
(eg, iostream) should be included using the '#include <iostream>' form
while header files for the classes I write should be included as '#include
"myheader.h"'.

Both forms cause implementation-defined behaviour. Whatever you know
about what actually happens is specific to your compiler. Another
compiler is free to behave differently. It has to document it, however.

I'm not so sure though for the cases in between. For example what if I
purchase a third party library. Which form should I use to include its
headers? Are there any hard and fast rules? Any conventions? Thanks in
advance for any replies.

It's all driven by the compiler rules. Read the documentation for your
compiler, it should explain those details. Most compilers I used did
have <> for searching in "predefined places", but VC++, for example, will
search for "" in those places, too.

Recently I've read some things about the differences, alleging that <>
(angle brackets) are reserved for the compiler headers. AFAICT it's not
necessarily so, and even if it is so in the Standard, all compilers I've
used do allow you to have your own header [file] and define its name in
the angle brackets. To me that means there is no rule saying that <>
should only name headers and never user's files.

V

 

[Exits Funnel]

Hello,

I'm slightly confused about when to use parens around #included files
and when to use angle brackets. I understand (I think) that the
difference is that the compiler will search in its standard include
directories for files included as <file> but not for those included as
"file". It's clear to me then that headers for the standard libraries
(eg, iostream) should be included using the '#include <iostream>' form
while header files for the classes I write should be included as
'#include "myheader.h"'.

I'm not so sure though for the cases in between. For example what if I
purchase a third party library. Which form should I use to include its
headers? Are there any hard and fast rules? Any conventions? Thanks
in advance for any replies.

-exits

 

[Jack Klein]

I'm slightly confused about when to use parens around #included files

You're not alone.

and when to use angle brackets. I understand (I think) that the
difference is that the compiler will search in its standard include
directories for files included as <file> but not for those included as
"file". It's clear to me then that headers for the standard libraries
(eg, iostream) should be included using the '#include <iostream>' form
while header files for the classes I write should be included as '#include
"myheader.h"'.

Both forms cause implementation-defined behaviour. Whatever you know
about what actually happens is specific to your compiler. Another
compiler is free to behave differently. It has to document it, however.

I'm not so sure though for the cases in between. For example what if I
purchase a third party library. Which form should I use to include its
headers? Are there any hard and fast rules? Any conventions? Thanks in
advance for any replies.

It's all driven by the compiler rules. Read the documentation for your
compiler, it should explain those details. Most compilers I used did
have <> for searching in "predefined places", but VC++, for example, will
search for "" in those places, too.

Recently I've read some things about the differences, alleging that <>
(angle brackets) are reserved for the compiler headers. AFAICT it's not
necessarily so, and even if it is so in the Standard, all compilers I've
used do allow you to have your own header [file] and define its name in
the angle brackets. To me that means there is no rule saying that <>
should only name headers and never user's files.

V

For the standard headers provided as part of the standard language,
such as <iostream> or <cstdlib>, the standard requires the use of <>,
and does not require that the standard headers be files.

This does not have much practical effect in any implementation that I
know of, but even if <iostream> is a file, these two directives are
not guaranteed or required to work the same way:

#include <iostream>


#include "iostream"

The second is not guaranteed to work properly by the standard, the
first is.

For inclusion of files, there is no real difference other than the
locations searched.

For a #include "somename" directive, the C++ standard requires that if
the file is not found in the special locations searched for this form
of the directive, the implementation must then search in whatever
other places it searches for the <> form of the include directive.

The real problem with putting standard headers in the "quoted" form is
that the compiler might stumble over some other file of the same name
somewhere in its search path before it comes to the real standard
header.

 

[Victor Bazarov]

Jack, Victor, thanks for taking the time to reply.

You've confirmed my understanding of the issue, but the question still
remains: which form do you use for 3rd party non-system headers? I
understand I can make it work with either form in conjuction with the
appropriate compiler switches but what is the standard usage?

There is no standard usage. The Standard leaves it to the implementation.
So, for every implementation you have to make the decision about what form
to use and what compiler switches to apply.

In other words, should one use the quotes for his own files and angle
brackets for everything else or should he use angle brackets for system
headers and quotes for everything else?

Up to you. The angle brackets are required for the standard headers.
Everything else is up to the implementation.

V

 

[Exits Funnel]

A lot of useful information snipped

Jack, Victor, thanks for taking the time to reply.

You've confirmed my understanding of the issue, but the question still
remains: which form do you use for 3rd party non-system headers? I
understand I can make it work with either form in conjuction with the
appropriate compiler switches but what is the standard usage? In other
words, should one use the quotes for his own files and angle brackets
for everything else or should he use angle brackets for system headers
and quotes for everything else?

Thanks again for taking the time to reply.

-exits