为什么要包含标头并向前声明同一 cpp 文件中包含的类？

发布于 2024-10-16 11:55:42 字数 344 浏览 9 评论 0原文

我一直在为我的大学项目查看 Fear SDK，但注意到一些代码，如下所示：

Foo.h

class Foo
{
    public:
        int iSomething;
};

Bar.cpp：

#include "Foo.h"

// Forward declarations
class Foo;

是否有任何特殊原因需要转发声明并在同一 cpp 文件中包含适当的标头？或者前向声明是否多余，因为包含了标头？

编辑：

每次我在代码中看到它时，包含语句总是位于前向声明之前。

原文

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.

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睫毛溺水了 2024-10-23 11:55:42

这不仅仅是多余的，而且还可能存在问题。假设 Foo.h 发生变化，那么 Foo 就会成为通用、模板化等价物的某些特定实例的 typedef - 这种事情可以预见为正常软件演化的一部分。那么 Bar.cpp 的“class X”将不必要地导致编译错误 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

这是我总是建议使用专用前向声明标头 ala 的原因之一，由主标头维护并包含在主标头中以确保持续的一致性。我从来没有把“X类；”在实现文件中，除非该类也在其中定义。请记住“X 级”看似的好处；前向声明并不是说它们避免了 #include，更重要的是它们包含的文件可能很大，进而包含许多其他文件：专用前向声明标头通常会避免绝大多数文件无论如何。

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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