为什么包含一个头文件并向前声明包含在同一个cpp文件中的类？

问题描述

我一直在为我的大学项目寻找恐惧SDK，但已经注意到一些类似的代码：

Foo.h

  class Foo 
 {
 public：
 int iSomething; 
}; 
  

Bar.cpp：

  #includeFoo.h
 
 //正向声明
 class Foo; 
  

是否有任何特别的理由来转发declare AND在相同的cpp文件中包含适当的头文件？或者是前面的声明是多余的，因为头文件被包含在内？

编辑：每次我有在代码中看到它，include声明总是在前向声明之前。

解决方案

它不仅仅是多余的，它可能有问题。说Foo.h发生了变化，所以Foo变成了一种泛型模板化等价物的特定实例化的类型定义 - 这是可以作为正常软件进化的一部分而预料到的东西。然后Bar.cpp的class X将不必要地导致编译错误ala：

  --- fwd.h --- 
模板< typename T> 
 class XT 
 {
 public：
 int n_; 
}; 
 
 typedef XT< int> X; 
 
 --- fwd.cc --- 
 #includefwd.h
 
 class X; 
 
 int main（）
 {
 X x; 
 x.n_ = 0; 
 return x.n_; 
} 
 
 ---编译尝试--- 
〜/ dev ... / gcc / 4.1.1 / exec / bin / g ++ fwd.cc -o fwd 
 fwd.cc:3：错误：在'class'后面使用typedef-name'X'
 fwd.h：8：错误：'X'在这里有一个前面的声明
  $ b 
这是我总推荐使用专用前向声明头文件ala < iosfwd>  #include ，更多的是它们包含的文件可能很大，并且包含许多其他文件：专用前向声明无论如何，标题通常会避免绝大多数。 
I've been looking at the Fear SDK for my university project, but have noticed some code like so:

Foo.h
class Foo
{
    public:
        int iSomething;
};
Bar.cpp:
#include "Foo.h"

// Forward declarations
class Foo;
Is there any particular reason to forward declare AND include the appropriate header in the same cpp file? Or is the forward declaration redundant because the header is being included?
EDIT:
Every time that I have seen it in the code, the include statement is always before the forward declaration.
 解决方案 
It's more than simply redundant, it's potentially problematic.  Say Foo.h changes so Foo becomes a typedef to some particular instantiation of a generic, templatised equivalent - the kind of thing that can be anticipated as part of normal software evolution.  Then Bar.cpp's "class X" will needlessly cause a compilation error ala:
--- fwd.h ---
template <typename T>
class XT
{
  public:
    int n_;
};

typedef XT<int> X;

--- fwd.cc ---
#include "fwd.h"

class X;

int main()
{
    X x;
    x.n_ = 0;
    return x.n_;
}

--- compilation attempt ---
~/dev  .../gcc/4.1.1/exec/bin/g++ fwd.cc -o fwd
fwd.cc:3: error: using typedef-name 'X' after 'class'
fwd.h:8: error: 'X' has a previous declaration here
This is one reason I always recommend using dedicated forward-declaration headers ala <iosfwd>, maintained with and included by the main header to ensure ongoing consistency.  I never put "class X;" in an implementation file unless the class is defined in there too.  Remember that the seeming benefits of "class X;" forward declarations is not so much that they avoid an #include, and more that the files they include can be large and in turn include a lot of other files: dedicated forward-declaration headers typically avoid the overwhelming majority of that anyway.
                        
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