枚举尚未声明？

Question

I'm getting this error. It's like the compiler does not recognise my declarations

 g++ -c main.cc
In file included from Storage.h:7:0,
                 from Server.h:5,
                 from Control.h:8,
                 from main.cc:5:
Serializer.h:11:36: error: ‘Storage::UpdateType’ has not been declared
Serializer.h:12:45: error: ‘Storage::UpdateType’ has not been declared
make: *** [main.o] Error 1

Anyone have an idea what this error is about because the enum has already beeen declared. The affected code is below:

Serializer.h

#ifndef SERIALIZER_H
#define SERIALIZER_H

#include "Storage.h"

class Storage;

class Serializer{
  public: 
    Serializer();
    void serialize(List&, Storage::UpdateType&, std::string&);
    void deserialize(std::string&, Storage::UpdateType&, List&);

};

#endif

Storage.h

#ifndef STORAGE_H
#define STORAGE_H 

#include "List.h"
#include "Interface.h"
#include "Movie.h"
#include "Serializer.h"

class Storage{
  public:
    enum UpdateType {ADD, DELETE, RETRIEVE};
    Storage();
    ~Storage();
    List* list;
    void retrieve(List*);
    void update(UpdateType, List*);
    void handleRequest(string&, string&);
  private:
    //Serializer serial;
};
#endif

Answer 1

The problem you have is that your code is interpreted as follows:

#ifndef STORAGE_H
#define STORAGE_H 

// replacing the include with the Serializer.h file

#include "List.h"
#include "Interface.h"
#include "Movie.h"

// replacing the include with the Storage.h file
#ifndef STORAGE_H // returns true
#endif

class Storage;

class Serializer{
  public: 
    Serializer();
    // here Storage::UpdateType is still unknown to the compiler
    void serialize(List&, Storage::UpdateType&, std::string&);
    void deserialize(std::string&, Storage::UpdateType&, List&);

};

#endif


class Storage{
  public:
    // as it gets declared here
    enum UpdateType {ADD, DELETE, RETRIEVE};
    Storage();
    ~Storage();
    List* list;
    void retrieve(List*);
    void update(UpdateType, List*);
    void handleRequest(string&, string&);
  private:
    //Serializer serial;
};
#endif

the best solution, imho, would be to extract the enum UpdateType from your storage class and forward declare it in Serializer. Or even declare it within the Serializer header, as Storage is encapsulating Serializer stuff, so basically, the UpdateType stuff should be in the same encapsulated context as the Serializer stuff.

Another solution would be @Nim suggests, to include Storage from Serializer, forward declare Serializer in Storage and include Serializer in main. But it may deceives the way you have thought your design out, reversing the encapsulation of Serializer and Storage .

Finally, I'm really not sure if this is possible and how would be the syntax, but if it is, you could simply forward declare enum Storage::UpdateType in Serializer. And I guess that's what you wish would be possible.