How can i prevent class redefinition problem ?

[ankurjain]

hi all,

suppose there are three files a.h,b.h,c.h
which contain classes a,b,c respectively.
and let c.h included in b.h and b.h included in a.h.

//c.h
 
class c
{ ....
};
//b.h
#include c.h
class c
{...
}
//a.h
#include b.h
#include c.h
class a
{...
}

//c.h

class c
{ ....
};
//b.h
#include c.h
class c
{...
}
//a.h
#include b.h
#include c.h
class a
{...
}

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On compiling there comes an error of class redeclaration ..... how can we prevent this ?

[ankurjain]

ok putting the b.h file in #ifndef ... #endif block solved the problem ...

[mcosta]

hi all,

suppose there are three files a.h,b.h,c.h
which contain classes a,b,c respectively.
and let c.h included in b.h and b.h included in a.h.

//c.h
 
class c
{ ....
};
//b.h
#include c.h
class c
{...
}
//a.h
#include b.h
#include c.h
class a
{...
}

//c.h

class c
{ ....
};
//b.h
#include c.h
class c
{...
}
//a.h
#include b.h
#include c.h
class a
{...
}

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On compiling there comes an error of class redeclaration ..... how can we prevent this ?

The standard way is

Generic header file

#ifndef <FILENAME>_H
#define <FILENAME>_H
 
class <ClassName>
{
};
#endif // <FILENAME>_H

#ifndef <FILENAME>_H
#define <FILENAME>_H

class <ClassName>
{
};
#endif // <FILENAME>_H

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In example

// a.h
#ifndef a_h
#define a_h
 
class a
{
 
};
#endif // a_h

// a.h
#ifndef a_h
#define a_h

class a
{

};
#endif // a_h

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// b.h
#ifndef b_h
#define b_h
 
#include "a.h"
 
class b
{
 
};
#endif // b_h

// b.h
#ifndef b_h
#define b_h

#include "a.h"

class b
{

};
#endif // b_h

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// c.h
#ifndef c_h
#define c_h
 
#include "a.h"
#include "b.h"
 
class c
{
 
};
#endif // c_h

// c.h
#ifndef c_h
#define c_h

#include "a.h"
#include "b.h"

class c
{

};
#endif // c_h

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In this way the second inclusion of "a.h" are ignored by precompiler.

[gfunk]

If you use Visual C++, you can also try:
#pragma once
at the top of each header file, this makes the compiler read the header just once.

[jacek]

If you use Visual C++, you can also try:
#pragma once
at the top of each header file, this makes the compiler read the header just once.

Beware that this solution isn't portable.

[iGoo]

Put all Class declaration in One generic header file.
such as:

//className.h
#ifndef CLASS_NAME_H
#define CLASS_NAME_H
class a;
class b;
class c;
//....
 
#endif //CLASS_NAME_H

//className.h
#ifndef CLASS_NAME_H
#define CLASS_NAME_H
class a;
class b;
class c;
//....

#endif //CLASS_NAME_H

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