C++ Enum inside a class - order of declaration matters? - c++

Question

I am using visual studio 2010, and can't quite get, how is this class wrong (syntax error : identifier 'EnumType ') and won't compile:
class BrokenClassWithEnum
{
private:
void useEnum (EnumType enumType); //syntax error : identifier 'EnumType '
public:
enum EnumType
{
VAL1,
VAL2,
VAL3
};
}
And this is OK:
class WorkingClassWithEnum
{
public:
enum EnumType
{
VAL1,
VAL2,
VAL3
};
private:
void useEnum (EnumType enumType);
}
What happened to the class scope?

Answer 1

I don't think the order of definition (not declaration) matters but a forward declaration will resolve this error - at least in MSVC++ 6. Outside of MSVC++ 6 you should specify a storage type to forward declare an enum (the requirement for specifying the storage type in a forward enum was in the C++0x standard)
NOTE: VC++ 6 will allow the storage type to be omitted, but if you are forward declaring an enum, you should declare the storage type (SEE BELOW):
This will resolve the error in VC++6. However, VC++6 does not appear to support a STORAGE TYPE for an enum inside a class as required by the C++0x standard:
class BrokenClassWithEnum {
public:
enum EnumType;
private:
void useEnum (EnumType enumType);
public:
enum EnumType {
VAL1,
VAL2,
VAL3
};
};
Normally I.E. within a C++0x compliant compiler you would use something like:
class BrokenClassWithEnum {
public:
enum EnumType : int;
private:
void useEnum (EnumType enumType);
public:
enum EnumType : int {
VAL1,
VAL2,
VAL3
};
};
NOTE:Forward declaration of enums is possible in some but not all C++ versions:
Forward declaring an enum in c++

Answer 2

In C++ any referenced non-built-in name must have been declared somewhere earlier.
It might appear that this rule has an exception for class definitions, because the following works fine:
struct S
{
void foo() { cout << x_ << endl; }
int x_;
S(): x_( 42 ) {}
};
However, the rule about declaration-before-use applies to the transformed code
struct S
{
inline void foo();
int x_;
inline S();
};
void S::foo() { cout << x_ << endl; }
S::S() : x_( 42 ) {}
which is what the compiler "proper" sees. And here there is no use of anything that hasn't already been declared.
The proper C++03 solution to your problem is to define the enumeration type before its first use. With C++11 you can alternatively forward-declare it, but then an underlying type must be specified,
C++11 §7.2/3:
“An opaque-enum-declaration is either a redeclaration of an enumeration in the current scope or a declaration
of a new enumeration. [Note: An enumeration declared by an opaque-enum-declaration has fixed underlying
type and is a complete type. The list of enumerators can be provided in a later redeclaration with an enum-
specifier. —end note ] A scoped enumeration shall not be later redeclared as unscoped or with a different
underlying type. An unscoped enumeration shall not be later redeclared as scoped and each redeclaration
shall include an enum-base specifying the same underlying type as in the original declaration.”

Answer 3

Within the class definition itself, items must appear in order (with dependent items appearing before the items that depend on them) like they do at any other scope. Inside a class method body (even if inlined) the entire definition of the class is available.

Answer 4

I believe member variables can be accessed before declarations within the class because class validation or compilation is done in two pass.