Another C++ was not declared in this scope error

Several questions has been asked related to this error, but each one of them practically relates to the object or type in question not declared before usage. For example:

 class A 
 {
   public:
 开发者_StackOverflow中文版    A_Object a_obj;   
 };

Getting the error A_Object was not declared in this scope means A_object is not declared anywhere within the file.

NOTE: This is my understanding of the error.

Now I have a file called Account.h as shown below:

#ifndef ACCOUNT_H_
#define ACCOUNT_H_
class Account
{
  //fields and methods
};
#endif /* ACCOUNT_H_ */

I also have a second file called Address.h as shown below:

#ifndef ADDRESS_H_
#define ADDRESS_H_
#include "Account.h"

typedef Account account_type;//Error here
class Address
{
 //Fields and methods
};
#endif /* ADDRESS_H_ */

When I try to compile this file I get the error Account was not declared in this scope.

Any Ideas why?

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Does Account.h actually also include Address.h? Such a circular reference seems the most likely situation.

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Do you have a matching #endif at the end of both include files?

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First point, your understanding about A_object is incorrect, the error means that A_object was not declared prior to it's first use, not that it wasn't declared anywhere.

Second point, the code you posted is incorrect, because you are missing #endif from both files. But assuming that was the only missing code then you would not get the error you describe. Post the real code that has the error.

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I've seen this error when Address.h includes Account.h, which includes OtherFile.h, which includes Address.h. Is it possible you have a circular dependency? It may be hard to find.

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This might be a case where a more core understanding of how the c/c++ compiler works would be in order. Include blocks, forward declarations, includes etc. All of these concepts did not make sense to me until I understood the basics of how compiler works. While I realize that this is somewhat of an oversimplification of compiler theory / logic, bear with me.

One of the first steps that a c++ compiler performs is a pre-processing (pre-compiler) step where it takes all of the files that are required, and combines them into one big flat file. In "C" languages, these pre-compiler operations a denoted using the hash (#) symbol. All an "#include" is doing, is directing the pre-compiler to bring this file into the entire "flat-file". If you have a cyclic include, your pre-compiler will get into an infinite loop and blow up, or say something super generic and useful like "the symbol has already been defined".

Include blocks, forward declarations, and all of the neat things that you are taught in c++ books that say "just do it, trust me" generally are helping you to avoid these type of compiling problems.