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Le Chaud Lapin
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 Posted: Fri Apr 15, 2005 7:27 am Post subject: Using Enums As Cheap Namespaces |
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I'm probably going to get castigated for my ignorance, but here goes:
I just used Visual Studio 7.0 (what is this thing really called anyway)
to compile a large project that had been compiling "flawlessly" under
Visual Studio 6.0. With the new compiler, I have no less than 700
errors, and they all seem to be related to my improper use of enums as
cheap namespaces. In a .hpp file I write:
enum Service_Code {COPY, DELETE, SEARCH,...} ;
In a .cpp file I use COPY, qualifying it with Service_Code so that VS
6.0 will not complain:
pier << Service_Code::COPY;
VS 6.0 compiles this with no errors. VS 7.0 complains:
LibraryFile.cpp(117) : error C2825:
'TDS::Waiter_File::Service_Code::COPY': cannot form a qualified name
LibraryFile.cpp(117) : error C2039: 'COPY' : is not a member of
'operator``global namespace'''
Could someone explain to me why there is a disparity, whether it should
or should not work, and what is the proper (portable and conforming)
way to achieve the same result with the same convenience?
Thanks. :)
-Le Chaud Lapin-
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Maxim Yegorushkin
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| Posted: Fri Apr 15, 2005 9:22 am Post subject: Re: Using Enums As Cheap Namespaces |
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On 15 Apr 2005 03:27:16 -0400, Le Chaud Lapin
<unoriginal_username (AT) yahoo (DOT) com> wrote:
| Quote: | enum Service_Code {COPY, DELETE, SEARCH,...} ;
In a .cpp file I use COPY, qualifying it with Service_Code so that VS
6.0 will not complain:
pier << Service_Code::COPY;
|
As enum is a C legacy stuff, I'm pretty much sure they should not
introduce name scopes. Although in VC6 you access them like
Service_Code::COPY, COPY is still hanging in the enum's enclosing scope
accessible without Service_Code::.
--
Maxim Yegorushkin
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Thomas Maeder
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| Posted: Fri Apr 15, 2005 9:26 am Post subject: Re: Using Enums As Cheap Namespaces |
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"Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> writes:
| Quote: | In a .hpp file I write:
enum Service_Code {COPY, DELETE, SEARCH,...} ;
In a .cpp file I use COPY, qualifying it with Service_Code so that VS
6.0 will not complain:
pier << Service_Code::COPY;
VS 6.0 compiles this with no errors. VS 7.0 complains:
LibraryFile.cpp(117) : error C2825:
'TDS::Waiter_File::Service_Code::COPY': cannot form a qualified name
LibraryFile.cpp(117) : error C2039: 'COPY' : is not a member of
'operator``global namespace'''
Could someone explain to me why there is a disparity, whether it should
or should not work, and what is the proper (portable and conforming)
way to achieve the same result with the same convenience?
|
It should not work. For VS 7.0, they seem to have fixed a bug Vs 6.0
had.
You can use
namespace Service_Code
{
enum
{
COPY,
DELETE,
SEARCH
};
}
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ThosRTanner
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| Posted: Fri Apr 15, 2005 9:26 am Post subject: Re: Using Enums As Cheap Namespaces |
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The name inside an enum gets yanked up to the containing level, so when
you do
class Wibble
{
enum Fruit { Apple, Orange, Banana }
}
You have an enum Wibble::Fruit, and enumerations Wibble::Apple,
Wibble::Orange, Wibble::Banana.
This is for compatibilty with C.
VC6 was incorrect in allowing your code through. I'm not sure what the
best way to deal with your issue is, choices being:
1) To change the enum to a namespace containing the enum. That will
work only if you don't use the enum name.
2) Change the enum to a class containing the enum (as per wibble
above).
3) Wait for the extension to C++ to be approved which makes it possible
to make enums behave like you (and I!) want.
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Stefan Näwe
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| Posted: Fri Apr 15, 2005 9:27 am Post subject: Re: Using Enums As Cheap Namespaces |
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Le Chaud Lapin schrieb:
| Quote: | enum Service_Code {COPY, DELETE, SEARCH,...} ;
In a .cpp file I use COPY, qualifying it with Service_Code so that VS
6.0 will not complain:
pier << Service_Code::COPY;
VS 6.0 compiles this with no errors. VS 7.0 complains:
LibraryFile.cpp(117) : error C2825:
'TDS::Waiter_File::Service_Code::COPY': cannot form a qualified name
LibraryFile.cpp(117) : error C2039: 'COPY' : is not a member of
'operator``global namespace'''
Could someone explain to me why there is a disparity, whether it should
or should not work, and what is the proper (portable and conforming)
way to achieve the same result with the same convenience?
|
enums are not namespaces. You have to use the enum values unqualified:
pier << COPY;
or, if the enum is defined inside a class/namespace (as in your case, I
guess):
pier << TDS::Waiter_File::COPY;
HTH
Stefan
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Isaac Rodriguez
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| Posted: Sat Apr 16, 2005 12:06 am Post subject: Re: Using Enums As Cheap Namespaces |
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| Quote: | As enum is a C legacy stuff, I'm pretty much sure they should not
introduce name scopes. Although in VC6 you access them like
Service_Code::COPY, COPY is still hanging in the enum's enclosing scope
accessible without Service_Code::.
|
Is there any effort on changing this for the next standard? It is
contra-intuitive the way it works now because of the syntax. The syntax is
similar to a struct, class, or namespace declaration, which they all
introduce a new scope, then you deal with enums, and they don't.
Also, enums do not really create a new type. There have been some instances
when I wished they did create a new type, so you can overload functions on
them or have some extra checking at compile time. I got a say that some
times it has been nice that they do not introduce a type, but I see some
type checking advantages if enums declared a new type that I think I could
live with its disadvantages.
Thanks,
- Isaac.
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Markus Moll
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| Posted: Sat Apr 16, 2005 5:44 pm Post subject: Re: Using Enums As Cheap Namespaces |
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Isaac Rodriguez wrote:
| Quote: | As enum is a C legacy stuff
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Imho enums are an important part of the language.
(See Java < 1.5, which lacked enums... pain in the...)
| Quote: | The syntax is
similar to a struct, class, or namespace declaration, which they all
introduce a new scope, then you deal with enums, and they don't.
|
http://www.research.att.com/~bs/evol-issues.html
ES029, ES030
I'd really appreciate scoped enumerators, but I think we won't see them as
suggested in ES029, because it would break C code. (C compatibility _is_ an
annoying feature) Maybe we will get a combined explicit/scoped enumerator.
| Quote: | Also, enums do not really create a new type.
|
What do you mean?
Is your compiler broken?
"[3.9.2 Compound types]
[...]
- enumerations, which comprise a set of named constant values. Each distinct
enumeration constitutes a different enumerated type, 7.2;"
and
"[7.2.4]
Each enumeration defines a type that is different from all other types.
[...]"
Markus
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Seungbeom Kim
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| Posted: Sat Apr 16, 2005 5:48 pm Post subject: Re: Using Enums As Cheap Namespaces |
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Isaac Rodriguez wrote:
| Quote: | Also, enums do not really create a new type. There have been some instances
when I wished they did create a new type, so you can overload functions on
them or have some extra checking at compile time. I got a say that some
times it has been nice that they do not introduce a type, but I see some
type checking advantages if enums declared a new type that I think I could
live with its disadvantages.
|
They do create a new type; you can overload functions on them or have some
extra checking at compile time.
enum RGB {R, G, B};
enum CMY {C, M, Y};
void f(RGB x) { std::clog << "f(RGB " << x << ")n"; }
void f(CMY x) { std::clog << "f(CMY " << x << ")n"; }
int main()
{
RGB rgb = R;
CMY cmy = C;
f(rgb);
f(cmy); // They do overload
f(0); // ERROR: No implicit conversion
f(static_cast
}
--
Seungbeom Kim
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Maxim Yegorushkin
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| Posted: Sun Apr 17, 2005 2:23 pm Post subject: Re: Using Enums As Cheap Namespaces |
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On 15 Apr 2005 20:06:09 -0400, Isaac Rodriguez
<discussion.support (AT) autodesk (DOT) com> wrote:
[]
| Quote: | Also, enums do not really create a new type. There have been some
instances
when I wished they did create a new type, so you can overload functions
on
them or have some extra checking at compile time. I got a say that some
times it has been nice that they do not introduce a type, but I see some
type checking advantages if enums declared a new type that I think I
could live with its disadvantages.
|
Enums do introduce new types and you can overload operators and functions
for them.
enum E { one, two };
E operator++(E);
void f(int);
void f(E);
int main()
{
f(++one);
}
--
Maxim Yegorushkin
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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