diff dep poi nta obj ssi always *** function

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** 4) The "right way" to include **
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Classes you create will often have dependencies on other classes. A derived class, for example, will always be dependent on its parent, because in order to be derived from the parent, it must be aware of its parent at compile time.

There are two basic kinds of dependencies you need to be aware of:
1) stuff that can be forward declared
2) stuff that needs to be #included

If, for example, class A uses class B, then class B is one of class A‘s dependencies. Whether it can be forward declared or needs to be included depends on how B is used within A:


- do nothing if: A makes no references at all to B
- do nothing if: The only reference to B is in a friend declaration
- forward declare B if: A contains a B pointer or reference: B* myb;
- forward declare B if: one or more functions has a B object/pointer/reference
as a parementer, or as a return type: B MyFunction(B myb);
- #include "b.h" if: B is a parent class of A
- #include "b.h" if: A contains a B object: B myb;

You want to do the least drastic option possible. Do nothing if you can, but if you can‘t, forward declare if you can. But if it‘s necessary, then #include the other header.

Ideally, the dependencies for the class should be layed out in the header. Here is a typical outline of how a "right way" header might look:

myclass.h

//=================================
// include guard
#ifndef __MYCLASS_H_INCLUDED__
#define __MYCLASS_H_INCLUDED__

//=================================
// forward declared dependencies
class Foo;
class Bar;

//=================================
// included dependencies
#include <vector>
#include 
 
"parent.h"

//=================================
// the actual class
class MyClass : public Parent  // Parent object, so #include "parent.h"
{
public:
  std::vector<int> avector;    // vector object, so #include <vector>
  Foo* foo;                    // Foo pointer, so forward declare Foo
  void Func(Bar& bar);         // Bar reference, so forward declare Bar

  friend class MyFriend;       // friend declaration is not a dependency
                               //   don‘t do anything about MyFriend
};

#endif // __MYCLASS_H_INCLUDED__ 

This shows the two different kinds of dependencies and how to handle them. Because MyClass only uses a pointer to Foo and not a full Foo object, we can forward declare Foo, and don‘t need to #include "foo.h". You should always forward declare what you can -- don‘t #include unless it‘s necessary. Unnecessary #includes can lead to trouble.

If you stick to this system, you will bulletproof yourself, and will minimize #include related hazards.

mark一篇講C++的 .h 和 #include 的好文章，貼一道鏈接慢慢看

http://www.cplusplus.com/forum/articles/10627/

[Js-C++].h文件與#include詳解