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vivekian
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 Posted: Sun Nov 27, 2005 2:00 pm Post subject: How to include header files ? |
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Have a program where an object A uses a pointer to object B in a.cpp
and object B uses object A in b.cpp. The declarations have been put in
a.h and b.h respectively. Now if i include a.h and b.h both in a.cpp
and b.cpp , i get 'previously defined here' and 'redifination' errors.
What is the solution to this ?
Thanks,
vivekian
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Alf P. Steinbach
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| Posted: Sun Nov 27, 2005 2:13 pm Post subject: Re: How to include header files ? |
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* vivekian:
| Quote: | Have a program where an object A uses a pointer to object B in a.cpp
and object B uses object A in b.cpp. The declarations have been put in
a.h and b.h respectively. Now if i include a.h and b.h both in a.cpp
and b.cpp , i get 'previously defined here' and 'redifination' errors.
What is the solution to this ?
|
There are many solutions, not one.
FAQ item 39.11 gives a purely technical answer, perhaps the simplest, a
forward-declaration.
<url:
http://www.parashift.com/c++-faq-lite/misc-technical-issues.html#faq-39.11>.
struct A;
struct B;
struct A { B* myB; };
struct B { A* myA; };
It's important to know about forward declarations.
But an often more clean solution is the abstract class solution:
struct AbstractA { ... };
struct AbstractB { ... };
struct A: AbstractA { AbstractB* myB; };
struct B: AbstractB { AbstractA* myA; }
This solution helps you factor out what's really needed for A and B to
do their work, i.e. it solves the design-level problem instead of just
alleviating the immediate C++ symptom of the problem.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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vivekian
Guest
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| Posted: Sun Nov 27, 2005 3:34 pm Post subject: Re: How to include header files ? |
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Alf P. Steinbach wrote :
Not too sure about this , but i think by including those header files
a.h and b.h in both a.cpp and b.cpp i do forward declare them . The
problem i am facing not circular dependency but rather much more of
redeclaration compile time errors i.e.
// sample code - a.cpp
#include b.h
#include a.h
/*..........*/
//sample code - b.cpp
#include a.h
#include b.h
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Alf P. Steinbach
Guest
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| Posted: Sun Nov 27, 2005 4:00 pm Post subject: Re: How to include header files ? |
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* vivekian:
| Quote: | Alf P. Steinbach wrote :
There are many solutions, not one.
FAQ item 39.11 gives a purely technical answer, perhaps the simplest, a
forward-declaration.
url:
http://www.parashift.com/c++-faq-lite/misc-technical-issues.html#faq-39.11>.
struct A;
struct B;
struct A { B* myB; };
struct B { A* myA; };
It's important to know about forward declarations.
Not too sure about this , but i think by including those header files
a.h and b.h in both a.cpp and b.cpp i do forward declare them . The
problem i am facing not circular dependency but rather much more of
redeclaration compile time errors i.e.
// sample code - a.cpp
#include b.h
#include a.h
/*..........*/
//sample code - b.cpp
#include a.h
#include b.h
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Uh, well, then you just haven't come to the dependency problem yet.
For the redefinition problem just add proper #include guards to the
header files, like this:
#ifndef A_H
#define A_H
// ... contents of [a.h]
#endif
For more detailed information see section 6 through 10 of chapter
(whatever) 1.6 of the attempted "Correct C++ Tutorial", at
<url: http://home.no.net/dubjai/win32cpptut/html/w32cpptut_01.html>.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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vivekian
Guest
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| Posted: Sun Nov 27, 2005 5:02 pm Post subject: Re: How to include header files ? |
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Alf P. Steinbach wrote:
| Quote: | Uh, well, then you just haven't come to the dependency problem yet.
For the redefinition problem just add proper #include guards to the
header files, like this:
#ifndef A_H
#define A_H
// ... contents of [a.h]
#endif
For more detailed information see section 6 through 10 of chapter
(whatever) 1.6 of the attempted "Correct C++ Tutorial", at
url: http://home.no.net/dubjai/win32cpptut/html/w32cpptut_01.html>.
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Alf,
Thanks. Things much more clear now .
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EventHelix.com
Guest
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| Posted: Sun Nov 27, 2005 5:09 pm Post subject: Re: How to include header files ? |
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| Quote: | Have a program where an object A uses a pointer to object B in a.cpp
and object B uses object A in b.cpp. The declarations have been put in
a.h and b.h respectively. Now if i include a.h and b.h both in a.cpp
and b.cpp , i get 'previously defined here' and 'redifination' errors.
What is the solution to this ?
|
The following article should help:
http://www.eventhelix.com/RealtimeMantra/HeaderFileIncludePatterns.htm
--
EventStudio System Designer 2.5 - http://www.EventHelix.com/EventStudio
Sequence Diagram Based System Design and Object Modeling Tool
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