How to declare interdependent classes in different header files

[Andre]

Is it possible to declare interdependent classes in different header
files?

If so, how could the following errors in the following piece of code
be corrected?

Thanks in advance,

Andre

1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C2146:
syntax error : missing ';' before identifier 'mStructA'
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int
1>c:\users\andre\_my\tmp\test01\test01\Struct_A.h(8): error C2236:
unexpected 'class' 'Struct_A'. Did you forget a ';'?
1>c:\users\andre\_my\tmp\test01\test01\Struct_A.h(8): error C2143:
syntax error : missing ';' before '{'
1>c:\users\andre\_my\tmp\test01\test01\Struct_A.h(8): error C2447:
'{' : missing function header (old-style formal list?)

In Struct_A.hpp:

#ifndef STRUCT_A
#define STRUCT_A

#include "Struct_B.h"

class Struct_A
{
public:
Struct_B mStructB;
}

#endif // STRUCT_A

In Struct_B.hpp:

#ifndef STRUCT_B
#define STRUCT_B

#include "Struct_A.h"

class Struct_B
{
public:
Struct_A mStructA;
}

#endif // STRUCT_B

 

[Andre]

Is it possible to declare interdependent classes in different header
files?

If so, how could the following errors in the following piece of code
be corrected?

Thanks in advance,

Andre

1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C2146:
syntax error : missing ';' before identifier 'mStructA'
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int
1>c:\users\andre\_my\tmp\test01\test01\Struct_A.h(8): error C2236:
unexpected 'class' 'Struct_A'. Did you forget a ';'?
1>c:\users\andre\_my\tmp\test01\test01\Struct_A.h(8): error C2143:
syntax error : missing ';' before '{'
1>c:\users\andre\_my\tmp\test01\test01\Struct_A.h(8): error C2447:
'{' : missing function header (old-style formal list?)

In Struct_A.hpp:

#ifndef STRUCT_A
#define STRUCT_A

#include "Struct_B.h"

class Struct_A
{
public:
   Struct_B mStructB;

}

#endif // STRUCT_A

In Struct_B.hpp:

#ifndef STRUCT_B
#define STRUCT_B

#include "Struct_A.h"

class Struct_B
{
public:
   Struct_A mStructA;

}

#endif // STRUCT_B

Actually I made a mistake when declaring the classes. Please see
attached error messages and code

1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C2146:
syntax error : missing ';' before identifier 'mStructA'
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int

In Struct_A.hpp:

#ifndef STRUCT_A
#define STRUCT_A

#include "Struct_B.h"

class Struct_A
{
public:
Struct_B mStructB;
};

#endif // STRUCT_A

In Struct_B.hpp:

#ifndef STRUCT_B
#define STRUCT_B

#include "Struct_A.h"

class Struct_B
{
public:
Struct_A mStructA;
};

#endif // STRUCT_B

 

[Ben Bacarisse]

Actually I made a mistake when declaring the classes.

You want comp.lang.c++ no comp.lang.c.

<snip>

 

[John Bode]

Is it possible to declare interdependent classes in different header
files?

[snip]


Actually I made a mistake when declaring the classes. Please see
attached error messages and code

1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C2146:
syntax error : missing ';' before identifier 'mStructA'
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int
1>c:\users\andre\_my\tmp\test01\test01\Struct_B.h(10): error C4430:
missing type specifier - int assumed. Note: C++ does not support
default-int

In Struct_A.hpp:

#ifndef STRUCT_A
#define STRUCT_A

#include "Struct_B.h"

class Struct_A
{
public:
   Struct_B mStructB;

};

#endif // STRUCT_A

In Struct_B.hpp:

#ifndef STRUCT_B
#define STRUCT_B

#include "Struct_A.h"

class Struct_B
{
public:
   Struct_A mStructA;

};

#endif // STRUCT_B

You cannot have circular dependencies where type B has an instance of
type A and vice versa (this is equally true in C and C++). What you
can do is create incomplete type definitions for B and have A declare
a pointer (or in C++, a reference) to it and vice versa:

structA.h:
/**
* This example is C-based, but the same concept holds true
* for classes in C++
*/
#ifndef STRUCT_A
#define STRUCT_A

struct b; /* incomplete type definition */

struct a
{
struct b *bptr;
...
};

struct a afoo(struct b *ptr);
#endif

structB.h:

#ifndef STRUCT_B
#define STRUCT_B

struct a; /* incomplete type definition */

struct b
{
struct a *aptr;
...
};

struct b bfoo(struct a *ptr);
#endif

Note that neither header imports the other; instead, you defer that to
your implementation files, which will import both headers:

structA.c:

#include "structA.h"
#include "structB.h"

struct a afoo(struct b *ptr)
{
struct a newfoo;
newfoo.bptr = ptr;
return newfoo;
}

structB.c:

#include "structB.h"
#include "structA.h"

struct b bfoo(struct a *ptr)
{
struct b newfoo;
newfoo.aptr = ptr;
return newfoo;
}

Circular dependencies are bad juju and should be avoided whenever
possible. If you need to associate an A with a B or vice versa, it
might be better to create a third type to handle the mapping:

structC.h:

#ifndef STRUCT_C
#define STRUCT_C

#include "structA.h"
#include "structB.h"

struct c {
struct a m_a;
struct b m_b;
};

....