C++ nested classes referencing each other
In C# I can have two nested classes refer to each other without problem:
public class CFGFile
{
class Setting
{
public Entry Ent;
}
class Entry
{
public Setting Setg;
}
}
However, trying the same thing in C++ causes problems:
class CFGFile
{
class Setting;
class Entry
{
Setting Setg;
};
class Setting
{
Entry Ent;
]
};
I get
"incomplete type not allowed"
at the Setg variable definition, and error
"C2079: 'CFGFile::Entry::Setg' uses undefined class
'CFGFile::Setting'"
when compiling.
I'm using Visual Studio 2017.
Is cross referring not possible in nested classes in C++?
c++ class nested visual-studio-2017
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
asked Nov 21 '18 at 14:26
L Fitz
284
add a comment |
In C# I can have two nested classes refer to each other without problem:
public class CFGFile
{
class Setting
{
public Entry Ent;
}
class Entry
{
public Setting Setg;
}
}
However, trying the same thing in C++ causes problems:
class CFGFile
{
class Setting;
class Entry
{
Setting Setg;
};
class Setting
{
Entry Ent;
]
};
I get
"incomplete type not allowed"
at the Setg variable definition, and error
"C2079: 'CFGFile::Entry::Setg' uses undefined class
'CFGFile::Setting'"
when compiling.
I'm using Visual Studio 2017.
Is cross referring not possible in nested classes in C++?
c++ class nested visual-studio-2017
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
asked Nov 21 '18 at 14:26
L Fitz
284
3
Try with struct instead of class in C#, this should be a fair comparison. Shouldn't work either.
– tkausl
Nov 21 '18 at 14:27
@tkausl because in the C# example above, references to objects are being defined, not objects of the class types. I get it. Thanks!
– L Fitz
Nov 21 '18 at 14:46
values cannot beNULL, so what you wrote is aEntrycontaining aSettingwhich in turn contains anEntrywhich again contains aSettingwhich in turn contains anEntry.... ad infinitum
– user463035818
Nov 21 '18 at 15:02
add a comment |
In C# I can have two nested classes refer to each other without problem:
public class CFGFile
{
class Setting
{
public Entry Ent;
}
class Entry
{
public Setting Setg;
}
}
However, trying the same thing in C++ causes problems:
class CFGFile
{
class Setting;
class Entry
{
Setting Setg;
};
class Setting
{
Entry Ent;
]
};
I get
"incomplete type not allowed"
at the Setg variable definition, and error
"C2079: 'CFGFile::Entry::Setg' uses undefined class
'CFGFile::Setting'"
when compiling.
I'm using Visual Studio 2017.
Is cross referring not possible in nested classes in C++?
c++ class nested visual-studio-2017
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
asked Nov 21 '18 at 14:26
L Fitz
284
In C# I can have two nested classes refer to each other without problem:
public class CFGFile
{
class Setting
{
public Entry Ent;
}
class Entry
{
public Setting Setg;
}
}
However, trying the same thing in C++ causes problems:
class CFGFile
{
class Setting;
class Entry
{
Setting Setg;
};
class Setting
{
Entry Ent;
]
};
I get
"incomplete type not allowed"
at the Setg variable definition, and error
"C2079: 'CFGFile::Entry::Setg' uses undefined class
'CFGFile::Setting'"
when compiling.
I'm using Visual Studio 2017.
Is cross referring not possible in nested classes in C++?
c++ class nested visual-studio-2017
c++ class nested visual-studio-2017
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
asked Nov 21 '18 at 14:26
L Fitz
284
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
asked Nov 21 '18 at 14:26
L Fitz
284
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
edited Nov 21 '18 at 15:23
Poul Bak
5,43331232
5,43331232
asked Nov 21 '18 at 14:26
L Fitz
284
asked Nov 21 '18 at 14:26
L Fitz
284
asked Nov 21 '18 at 14:26
L Fitz
284
284
3
Try with struct instead of class in C#, this should be a fair comparison. Shouldn't work either.
– tkausl
Nov 21 '18 at 14:27
@tkausl because in the C# example above, references to objects are being defined, not objects of the class types. I get it. Thanks!
– L Fitz
Nov 21 '18 at 14:46
values cannot beNULL, so what you wrote is aEntrycontaining aSettingwhich in turn contains anEntrywhich again contains aSettingwhich in turn contains anEntry.... ad infinitum
– user463035818
Nov 21 '18 at 15:02
add a comment |
3
Try with struct instead of class in C#, this should be a fair comparison. Shouldn't work either.
– tkausl
Nov 21 '18 at 14:27
@tkausl because in the C# example above, references to objects are being defined, not objects of the class types. I get it. Thanks!
– L Fitz
Nov 21 '18 at 14:46
values cannot beNULL, so what you wrote is aEntrycontaining aSettingwhich in turn contains anEntrywhich again contains aSettingwhich in turn contains anEntry.... ad infinitum
– user463035818
Nov 21 '18 at 15:02
3
3
Try with struct instead of class in C#, this should be a fair comparison. Shouldn't work either.
– tkausl
Nov 21 '18 at 14:27
Try with struct instead of class in C#, this should be a fair comparison. Shouldn't work either.
– tkausl
Nov 21 '18 at 14:27
@tkausl because in the C# example above, references to objects are being defined, not objects of the class types. I get it. Thanks!
– L Fitz
Nov 21 '18 at 14:46
@tkausl because in the C# example above, references to objects are being defined, not objects of the class types. I get it. Thanks!
– L Fitz
Nov 21 '18 at 14:46
values cannot be
NULL, so what you wrote is a Entry containing a Setting which in turn contains an Entry which again contains a Setting which in turn contains an Entry.... ad infinitum– user463035818
Nov 21 '18 at 15:02
values cannot be
NULL, so what you wrote is a Entry containing a Setting which in turn contains an Entry which again contains a Setting which in turn contains an Entry.... ad infinitum– user463035818
Nov 21 '18 at 15:02
add a comment |
3 Answers
3
active
oldest
votes
This has nothing to do with nested or not. In C++, you can not cross reference each other for two classes/structs like that. The workaround is that you use either pointer or reference which does not require a complete type definition. In your case, try the following,
class CFGFile
{
class Setting;
class Entry
{
Setting* Setg; // or std::unique_ptr<Setting> Setg;
};
class Setting
{
Entry Ent;
};
};
As suggested by @Ted Lyngmo, std::unique_ptr is the preferred way in modern C++ for applications.
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
Perhaps a suggestion to usestd::unique_ptr<Setting> Setg;would be good.
– Ted Lyngmo
Nov 21 '18 at 14:43
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
add a comment |
To expand on why this happens, this comes from the fact that when the compiler looks at a struct or class, it needs to be able to determine its exact size, among other things to be able to know how much memory it needs to allocate for instances of these.
In this case, the compiler can't determine the size of Entry because it only knows that a Setting class exists, but it doesn't know its size yet.
Using a pointer, i.e.:
class Setting;
class Entry
{
Setting* Setg;
};
actually solves this problem, because while the compiler still doesn't know anything about Setting, it still knows the size of a pointer (regardless of the pointee's type).
As already mentioned, the "nested" part has no impact on this.
answered Nov 21 '18 at 14:56
JBL
9,70333667
add a comment |
C# - by design - uses reference semantics for classes, thus the provided snippet does not cause circular value depency. OTH c++ prefers value semantics , which would -in turn- result in infinite recursion in the original sample code, if the standard did not ban that kind of code; As implicated in earlier replies, if reference semantics is needed, it should be explicitly expressed in terms of pointer/reference syntax.
answered Nov 21 '18 at 15:29
Red.Wave
72737
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
This has nothing to do with nested or not. In C++, you can not cross reference each other for two classes/structs like that. The workaround is that you use either pointer or reference which does not require a complete type definition. In your case, try the following,
class CFGFile
{
class Setting;
class Entry
{
Setting* Setg; // or std::unique_ptr<Setting> Setg;
};
class Setting
{
Entry Ent;
};
};
As suggested by @Ted Lyngmo, std::unique_ptr is the preferred way in modern C++ for applications.
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
Perhaps a suggestion to usestd::unique_ptr<Setting> Setg;would be good.
– Ted Lyngmo
Nov 21 '18 at 14:43
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
add a comment |
This has nothing to do with nested or not. In C++, you can not cross reference each other for two classes/structs like that. The workaround is that you use either pointer or reference which does not require a complete type definition. In your case, try the following,
class CFGFile
{
class Setting;
class Entry
{
Setting* Setg; // or std::unique_ptr<Setting> Setg;
};
class Setting
{
Entry Ent;
};
};
As suggested by @Ted Lyngmo, std::unique_ptr is the preferred way in modern C++ for applications.
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
Perhaps a suggestion to usestd::unique_ptr<Setting> Setg;would be good.
– Ted Lyngmo
Nov 21 '18 at 14:43
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
add a comment |
This has nothing to do with nested or not. In C++, you can not cross reference each other for two classes/structs like that. The workaround is that you use either pointer or reference which does not require a complete type definition. In your case, try the following,
class CFGFile
{
class Setting;
class Entry
{
Setting* Setg; // or std::unique_ptr<Setting> Setg;
};
class Setting
{
Entry Ent;
};
};
As suggested by @Ted Lyngmo, std::unique_ptr is the preferred way in modern C++ for applications.
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
This has nothing to do with nested or not. In C++, you can not cross reference each other for two classes/structs like that. The workaround is that you use either pointer or reference which does not require a complete type definition. In your case, try the following,
class CFGFile
{
class Setting;
class Entry
{
Setting* Setg; // or std::unique_ptr<Setting> Setg;
};
class Setting
{
Entry Ent;
};
};
As suggested by @Ted Lyngmo, std::unique_ptr is the preferred way in modern C++ for applications.
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
edited Nov 21 '18 at 14:50
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
answered Nov 21 '18 at 14:31
CS Pei
8,3801938
8,3801938
Perhaps a suggestion to usestd::unique_ptr<Setting> Setg;would be good.
– Ted Lyngmo
Nov 21 '18 at 14:43
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
add a comment |
Perhaps a suggestion to usestd::unique_ptr<Setting> Setg;would be good.
– Ted Lyngmo
Nov 21 '18 at 14:43
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
Perhaps a suggestion to use
std::unique_ptr<Setting> Setg; would be good.– Ted Lyngmo
Nov 21 '18 at 14:43
Perhaps a suggestion to use
std::unique_ptr<Setting> Setg; would be good.– Ted Lyngmo
Nov 21 '18 at 14:43
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
Using pointers for cross references. Thanks!
– L Fitz
Nov 21 '18 at 14:46
add a comment |
To expand on why this happens, this comes from the fact that when the compiler looks at a struct or class, it needs to be able to determine its exact size, among other things to be able to know how much memory it needs to allocate for instances of these.
In this case, the compiler can't determine the size of Entry because it only knows that a Setting class exists, but it doesn't know its size yet.
Using a pointer, i.e.:
class Setting;
class Entry
{
Setting* Setg;
};
actually solves this problem, because while the compiler still doesn't know anything about Setting, it still knows the size of a pointer (regardless of the pointee's type).
As already mentioned, the "nested" part has no impact on this.
answered Nov 21 '18 at 14:56
JBL
9,70333667
add a comment |
To expand on why this happens, this comes from the fact that when the compiler looks at a struct or class, it needs to be able to determine its exact size, among other things to be able to know how much memory it needs to allocate for instances of these.
In this case, the compiler can't determine the size of Entry because it only knows that a Setting class exists, but it doesn't know its size yet.
Using a pointer, i.e.:
class Setting;
class Entry
{
Setting* Setg;
};
actually solves this problem, because while the compiler still doesn't know anything about Setting, it still knows the size of a pointer (regardless of the pointee's type).
As already mentioned, the "nested" part has no impact on this.
answered Nov 21 '18 at 14:56
JBL
9,70333667
add a comment |
To expand on why this happens, this comes from the fact that when the compiler looks at a struct or class, it needs to be able to determine its exact size, among other things to be able to know how much memory it needs to allocate for instances of these.
In this case, the compiler can't determine the size of Entry because it only knows that a Setting class exists, but it doesn't know its size yet.
Using a pointer, i.e.:
class Setting;
class Entry
{
Setting* Setg;
};
actually solves this problem, because while the compiler still doesn't know anything about Setting, it still knows the size of a pointer (regardless of the pointee's type).
As already mentioned, the "nested" part has no impact on this.
answered Nov 21 '18 at 14:56
JBL
9,70333667
To expand on why this happens, this comes from the fact that when the compiler looks at a struct or class, it needs to be able to determine its exact size, among other things to be able to know how much memory it needs to allocate for instances of these.
In this case, the compiler can't determine the size of Entry because it only knows that a Setting class exists, but it doesn't know its size yet.
Using a pointer, i.e.:
class Setting;
class Entry
{
Setting* Setg;
};
actually solves this problem, because while the compiler still doesn't know anything about Setting, it still knows the size of a pointer (regardless of the pointee's type).
As already mentioned, the "nested" part has no impact on this.
answered Nov 21 '18 at 14:56
JBL
9,70333667
answered Nov 21 '18 at 14:56
JBL
9,70333667
answered Nov 21 '18 at 14:56
JBL
9,70333667
answered Nov 21 '18 at 14:56
JBL
9,70333667
9,70333667
add a comment |
add a comment |
C# - by design - uses reference semantics for classes, thus the provided snippet does not cause circular value depency. OTH c++ prefers value semantics , which would -in turn- result in infinite recursion in the original sample code, if the standard did not ban that kind of code; As implicated in earlier replies, if reference semantics is needed, it should be explicitly expressed in terms of pointer/reference syntax.
answered Nov 21 '18 at 15:29
Red.Wave
72737
add a comment |
C# - by design - uses reference semantics for classes, thus the provided snippet does not cause circular value depency. OTH c++ prefers value semantics , which would -in turn- result in infinite recursion in the original sample code, if the standard did not ban that kind of code; As implicated in earlier replies, if reference semantics is needed, it should be explicitly expressed in terms of pointer/reference syntax.
answered Nov 21 '18 at 15:29
Red.Wave
72737
add a comment |
C# - by design - uses reference semantics for classes, thus the provided snippet does not cause circular value depency. OTH c++ prefers value semantics , which would -in turn- result in infinite recursion in the original sample code, if the standard did not ban that kind of code; As implicated in earlier replies, if reference semantics is needed, it should be explicitly expressed in terms of pointer/reference syntax.
answered Nov 21 '18 at 15:29
Red.Wave
72737
C# - by design - uses reference semantics for classes, thus the provided snippet does not cause circular value depency. OTH c++ prefers value semantics , which would -in turn- result in infinite recursion in the original sample code, if the standard did not ban that kind of code; As implicated in earlier replies, if reference semantics is needed, it should be explicitly expressed in terms of pointer/reference syntax.
answered Nov 21 '18 at 15:29
Red.Wave
72737
answered Nov 21 '18 at 15:29
Red.Wave
72737
answered Nov 21 '18 at 15:29
Red.Wave
72737
answered Nov 21 '18 at 15:29
Red.Wave
72737
72737
add a comment |
add a comment |
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3
Try with struct instead of class in C#, this should be a fair comparison. Shouldn't work either.
– tkausl
Nov 21 '18 at 14:27
@tkausl because in the C# example above, references to objects are being defined, not objects of the class types. I get it. Thanks!
– L Fitz
Nov 21 '18 at 14:46
values cannot be
NULL, so what you wrote is aEntrycontaining aSettingwhich in turn contains anEntrywhich again contains aSettingwhich in turn contains anEntry.... ad infinitum– user463035818
Nov 21 '18 at 15:02