Compiler doesn't like my class C++. getting undeclared identifier error and more

Question

I created a class called "Message". I want to store Messages that are created with the class "Message" in a static vector array in a class named "MessageBox". The compiler tells me that Message doesn't exist, but the editor is telling me otherwise. Here are the files with the code:

"Message.h"

#pragma once
#include <iostream>
#include <string>
#include "Message Box.h"

namespace ATE {
    class Message
    {
    public:
        Message(std::string act, std::string ID, std::string IDtwo) { action = act, ID1 = ID, ID2 = IDtwo; }
        Message(std::string act, std::string ID) { action = act, ID1 = ID; }

        std::string action;
        std::string ID1;
        std::string ID2 = nullptr;

    };

}

"Message Box.h"

#pragma once
#include <string>
#include <vector>
#include "Message.h"

namespace ATE {
    class MessageBox
    {
    public:
        static std::vector<Message> MsgBox;
        void addMessage(Message msg);

    };
}

"Message Box.cpp"

#include "Message Box.h"

void ATE::MessageBox::addMessage(Message msg)
{
     MsgBox.push_back(msg);
}

My errors:

Error C2065 'Message': undeclared identifier (file: message box.h, line: 11)

Error C2923 'std::vector': 'Message' is not a valid template type argument for parameter '_Ty' (file: message box.h, line: 11)

Error C2061 syntax error: identifier 'Message' (file: message box.h, line: 12)

Help is much appreciated (:

"Message.h" #pragma once // do you forget pragma once? #include <string> // #include "Message Box.h" // why do you include message box in the message.h file? ... /// std::string ID2 = nullptr; // what do you mean? — AnatolyS
yeah, i had the pragma once, but i forgot to copy it into this question. the #include "Message Box.h" in "Message.h" was from an old message system idea i had but then i started switching stuff around. std::string ID2 = nullptr is because it may never have a value. I took out the Message Box.h" in "Message.h" and now i'm getting link errors — NotHereAnymore
More in-depth discussion on what happened here: Resolve header include circular dependencies in C++ — user4581301
yeah, i hate refactoring code because i always have little bits left here and there. the 2nd part of the answer below got me out of the link errors. thank you all for your help — NotHereAnymore
You're compiling with Visual Studio, so be aware that windows.h has the nasty habit of #defining MessageBox. You might suddenly see an error on ATE::MessageBoxW, macro's do not respect scope. — MSalters

NonNumeric NonNumeric · Accepted Answer · 2016-06-22T20:59:31

Your headers include each other. Due to the #pragma once if you include Message.h first in some .cpp file the compiler will see Message Box.h's content "inside" Message.h - exactly at the #include "Message Box.h" line. As result the compiler will process your source code in the following order:

source.cpp:
  //#include "Message.h"
message.h:
  //#pragma once
  //#include <iostream>
iostream:
  //iostream's content
message.h(resumed):
  //#include <string>
string:
  //string's content
message.h(resumed):
  #include "Message Box.h"
Message Box.h:
  #pragma once
  #include <string>
  // string's content already included, won't include again
  #include <vector>
  // vector's content
  #include "Message.h"
  // message.h won't include again, due to the #pragma once

  namespace ATE {
      class MessageBox
      {
      public:
          static std::vector<Message> MsgBox;

At this point the name Message is used, but the compiler has not reached it's definition in Message.h yet and gives you an error.

Removing the #pragma once won't help. You need to remove the circular dependency. In this case remove the #include "Message Box.h" from Message.h and you'll be fine.

Compiler doesn't like my class C++. getting undeclared identifier error and more

2 Answers