C++ question

[Vertimus]

For simplicity, suppose I have two files, each with one namespace containing one class. One is "Aspen.h", the other "Birch.h"

Aspen.h contains:

#ifndef H_ASPEN
#define H_ASPEN

#include "Birch.h"

namespace Aspen {
class Alpha {
public:
void FunctionFoo(Birch::Beta*);
};
};
#endif

Birch.h contains:

#ifndef H_BIRCH
#define H_BIRCH

#include "Aspen.h"

namespace Birch {
class Beta {
public:
FunctionBar(int);
private:
Aspen::Alpha* m_data;
};
};
#endif

The problem:

When I compile "Aspen.cc" (not shown), the header "Aspen.h" includes "Birch.h". "Birch.h" has a private variable named m_data of type Aspen::Alpha*. However, the compiler (gcc 3.4) gives me the following errors:

In file included from Aspen.h:4:
Birch.h:11: error: `Aspen' has not been declared

But the file I'm compiling contains the namespace Aspen, so I have no clue to why it doesn't recognize Aspen.

Any remedies?

Thanks!

EDIT: sorry about the spacing. ATOT removes them.

 

[hypn0tik]

Shouldn't the line be:

include <Aspen.h>

Haven't programmed in C++ for a while but that's what I think it is.

Edit: Or perhaps:

#include <Aspen.h>

No clue.

 

[cliftonite]

Originally posted by: Vertimus
For simplicity, suppose I have two files, each with one namespace containing one class. One is "Aspen.h", the other "Birch.h"

Aspen.h contains:

#ifndef H_ASPEN
#define H_ASPEN

#include "Birch.h"

namespace Aspen {
class Alpha {
public:
void FunctionFoo(Birch::Beta*);
};
};
#endif

Birch.h contains:

#ifndef H_BIRCH
#define H_BIRCH

#include "Aspen.h"

namespace Birch {
class Beta {
public:
FunctionBar(int);
private:
Aspen::Alpha* m_data;
};
};
#endif

The problem:

When I compile "Aspen.cc" (not shown), the header "Aspen.h" includes "Birch.h". "Birch.h" has a private variable named m_data of type Aspen::Alpha*. However, the compiler (gcc 3.4) gives me the following errors:

In file included from Aspen.h:4:
Birch.h:11: error: `Aspen' has not been declared

But the file I'm compiling contains the namespace Aspen, so I have no clue to why it doesn't recognize Aspen.

Any remedies?

Thanks!

EDIT: sorry about the spacing. ATOT removes them.

Try declraing Aspen::Alpha* m_data; outside the function.

 

[MrChad]

Try the right forum.

 

[Vertimus]

Originally posted by: hypn0tik
Shouldn't the line be:

include <Aspen.h>

Haven't programmed in C++ for a while but that's what I think it is.

Edit: Or perhaps:

#include <Aspen.h>

No clue.

I thought triangular brackets were only used for system header files.

I changed to triangular brackets, and it now says it can't find "Birch.h", which matches my predictions.

 

[tami]

return false;

 

[hypn0tik]

Originally posted by: Vertimus

Originally posted by: hypn0tik
Shouldn't the line be:

include <Aspen.h>

Haven't programmed in C++ for a while but that's what I think it is.

Edit: Or perhaps:

#include <Aspen.h>

No clue.

I thought triangular brackets were only used for system header files.

I changed to triangular brackets, and it now says it can't find "Birch.h", which matches my predictions.

Ah, my bad. I don't know then. Sorry.

 

[Vertimus]

Originally posted by: cliftonite

Originally posted by: Vertimus
For simplicity, suppose I have two files, each with one namespace containing one class. One is "Aspen.h", the other "Birch.h"

Aspen.h contains:

#ifndef H_ASPEN
#define H_ASPEN

#include "Birch.h"

namespace Aspen {
class Alpha {
public:
void FunctionFoo(Birch::Beta*);
};
};
#endif

Birch.h contains:

#ifndef H_BIRCH
#define H_BIRCH

#include "Aspen.h"

namespace Birch {
class Beta {
public:
FunctionBar(int);
private:
Aspen::Alpha* m_data;
};
};
#endif

The problem:

When I compile "Aspen.cc" (not shown), the header "Aspen.h" includes "Birch.h". "Birch.h" has a private variable named m_data of type Aspen::Alpha*. However, the compiler (gcc 3.4) gives me the following errors:

In file included from Aspen.h:4:
Birch.h:11: error: `Aspen' has not been declared

But the file I'm compiling contains the namespace Aspen, so I have no clue to why it doesn't recognize Aspen.

Any remedies?

Thanks!

EDIT: sorry about the spacing. ATOT removes them.

Try declraing Aspen::Alpha* m_data; outside the function.

Outside what function?

I don't have any implementations of any functions, all i have are two declarations, none with Aspen::Alpha* m_data in them.

 

[akubi]

you need to forward declare Aspen before Birch.h is included.

Aspen.cc should start off with:

namespace Aspen {
class Aspen;
};
#include "Aspen.h"

this is necessary cuz you have a cyclic dependency betw aspen and birch

 

[cliftonite]

Originally posted by: Vertimus

Originally posted by: cliftonite

Originally posted by: Vertimus
For simplicity, suppose I have two files, each with one namespace containing one class. One is "Aspen.h", the other "Birch.h"

Aspen.h contains:

#ifndef H_ASPEN
#define H_ASPEN

#include "Birch.h"

namespace Aspen {
class Alpha {
public:
void FunctionFoo(Birch::Beta*);
};
};
#endif

Birch.h contains:

#ifndef H_BIRCH
#define H_BIRCH

#include "Aspen.h"

namespace Birch {
class Beta {
public:
FunctionBar(int);
private:
Aspen::Alpha* m_data;
};
};
#endif

The problem:

When I compile "Aspen.cc" (not shown), the header "Aspen.h" includes "Birch.h". "Birch.h" has a private variable named m_data of type Aspen::Alpha*. However, the compiler (gcc 3.4) gives me the following errors:

In file included from Aspen.h:4:
Birch.h:11: error: `Aspen' has not been declared

But the file I'm compiling contains the namespace Aspen, so I have no clue to why it doesn't recognize Aspen.

Any remedies?

Thanks!

EDIT: sorry about the spacing. ATOT removes them.

Try declraing Aspen::Alpha* m_data; outside the function.

Outside what function?

oops thought he declared inside bar

 

[Vertimus]

Originally posted by: akubi
you need to forward declare Aspen before Birch.h is included.

Aspen.cc should start off with:

namespace Aspen {
class Aspen;
};
#include "Aspen.h"

this is necessary cuz you have a cyclic dependency betw aspen and birch

Thanks, that seems to work

Is there a rule of thumb to when I need to forward declare?

 

[EagleKeeper]

Originally posted by: Vertimus
Is there a rule of thumb to when I need to forward declare?

Anytime you need a class that is not included via the header file.

Nested includes can easily create such cyclic dependencies.

 

[NikPreviousAcct]

Originally posted by: tami
return false;

teh win!

 

[oog]

Originally posted by: EagleKeeper

Originally posted by: Vertimus
Is there a rule of thumb to when I need to forward declare?

Anytime you need a class that is not included via the header file.

Nested includes can easily create such cyclic dependencies.

I would actually say it a little more strongly than EagleKeeper. I think you should use a forward declaration whenever you can do it. In general, the only time you really need to include another header file is when the compiler needs to know the size of the class you're referencing while compiling the current file. It can use the forward declaration for pointers, references and template parameters. It cannot use the forward declaration for member variables that are neither pointers nor references.

The reason for this stronger stance is that reducing the dependencies between your header files is just a good thing. Yes, it helps resolve circular dependencies, but it also allows other people to use your code without pulling in a whole bunch of things they may not need.