我试图实现一个组合关系，类似于 以下类型： class A { public： class B { int member_of_B; }; B b; // ...其他一些成员; }; 但由于B的定义太大我以为可能会把它写成 外面。 ）。所以我修改了课程： A级 { B级; B b; .... }; B级{ int member_of_B; }; 我们这里有两个问题： 1.包括.h在内的任何人（我已经宣布了这些课程） 将获得B类并可以使用它。我并不打算允许;）。 2. gcc 3.3.2给了我这种类型的错误： 错误：字段b具有不完整类型 我是否不允许使用正向引用？ 声明究竟是什么 B类; 的定义是什么？它不会在A中引入类型名吗？ 有什么建议吗？我错过了一些明显的东西吗？ ~M。
解决方案Milind写道： 我试图实现一个组合关系，以下类型的东西： A类 {公开： B级 { int member_of_B; }; B b; // ..其他一些成员; }; 但由于B的定义太大，我想到可能会在外面写它。 ）。所以我修改了课程： A级 {B / B级; B b; ... }; < B组{ int member_of_B; }; 我们这里有两个问题： 1.任何人包括 ; .h （我已经宣布了这些类）将得到类型B并且可以使用它。我并不打算允许;）。 2. gcc 3.3.2给了我这种类型的错误： 错误：字段`b''有不完整的类型 我不允许使用前向引用？ B类究竟是什么;在A的定义中做什么？它不是在A中引入了typename吗？ 这里有几个误解。前往 声明的正确方法是 B级; A级 { B b; }; B级 { }; 由于B类是在A类之外宣布的，因此前向声明 也必须在A类之外。 但第二个误解是，一个不完整的课程（在前瞻性申报和实际申报之间是B的B $ B $ 仅适用于极少数事情，例如声明指针。在你的代码中，编译器需要知道B的大小才能为A生成代码 ，但它不知道B的大小只是来自前向声明。 显而易见的解决办法就是颠倒你定义班级的顺序。 另一种解决方案是重写A这样的 B级; A级 { B * b; }; 通过使用指针，您不需要在 根据头文件。这也可能有助于您的其他要求 B本身无法使用。 有什么建议吗？我错过了一些明显的东西吗？
不明显，但我猜你是来自Java背景并且没有 相当吸收，但C ++中的对象是值而不是引用。轻松 Java和C ++之间最基本的区别。 john
>前进声明的正确方法是B类; A类 {b / b; }; B班 {}; 好吧即使这样也不会工作;（我复制粘贴上面的内容并尝试使用g ++ 3.3.2在我的Debian盒子上编译它。） /> 除此之外， 由于B级是在A级之外宣布的，因此前方声明也必须在A级之外。 根据我以前的帖子... 1.任何人包括.h......我并不打算允许;。 我不希望任何人直接使用B级，我总是希望其他人使用 它作为A :: B，即作为A类的一部分。以你建议的方式宣布建议将允许其他人按原样使用它。 使它成为一个成员肯定会强制使用B类对象。但是，我们不能使用裸B类物品来停止使用 。 然而第二个误解是一个不完整的类（这是 B在前向声明和实际声明之间是什么）只对很少的东西有用，例如声明指针。 是的;指针工作，因为他们有相同的大小，我们不分配内存。 我尝试b4发布在这里，并且它工作正常。这就是为什么我对b 对前向引用感到困惑的原因。如果类型已知，为什么不能编译器再做一次分配mem？ 编译器需要知道B的大小才能为A生成代码，但它不知道前向声明中B的大小。 这就是为什么我问：我不允许使用前向参考？ B类究竟是什么;在A的定义中做什么？它不是在A中引入了typename吗？ i意味着编译器不会再做第二遍以获得大小和 的东西吗？ 显而易见的解决方案就是颠倒你定义类的顺序。 嗯......但是如果我在A级之外定义B级，我会得到它 暴露给任何人，包括头文件。 A类 {B / B; }; 通过使用指针，您根本不需要在头文件中包含B的定义。这也可能有助于你的其他要求 B本身不可用。 我不知道如何通过在类中声明指针我可以限制 B'的用法。请问你好吗？ 我尝试用私人构造函数和朋友实现这个功能 函数。然而，必要的是在语义上意味着其他东西。 我想实现像 - >这样的东西。 A :: B总是。 有什么建议吗？我错过了一些明显的东西吗？
不明显
Thnx。因为我很担心:) john
Thnx。 ~M
可能''pimpl''成语可以帮助你： www.gotw.ca/publications/mill04.htm 再见 Fabio
Hi, I was trying to implement a composition relation, somthing of the following type: class A { public: class B { int member_of_B; }; B b; // ... some other members; }; but as the defintion of B is too large I thought of may be writing it outside. ;). So i modified the class as: class A { class B; B b; .... }; class B { int member_of_B; }; We have two issues here: 1. anybody including "the .h" (in which i have declared the classes) would get the type B and can use it. which i didn''t intend to allow ;). 2. gcc 3.3.2 gave me an error of this type: error: field `b'' has incomplete type Am I not allowed to use forward referencing ? What exactly does the statement class B; in definition of A do? Doesn''t it introduce the typename in A? Any suggestions? Am I missing something obvious ? ~ M.
解决方案 Milind wrote: Hi, I was trying to implement a composition relation, somthing of the following type: class A { public: class B { int member_of_B; }; B b; // ... some other members; }; but as the defintion of B is too large I thought of may be writing it outside. ;). So i modified the class as: class A { class B; B b; ... }; class B { int member_of_B; }; We have two issues here: 1. anybody including "the .h" (in which i have declared the classes) would get the type B and can use it. which i didn''t intend to allow ;). 2. gcc 3.3.2 gave me an error of this type: error: field `b'' has incomplete type Am I not allowed to use forward referencing ? What exactly does the statement class B; in definition of A do? Doesn''t it introduce the typename in A? A couple of misunderstandings here. The correct way to make the forward declaration is class B; class A { B b; }; class B { }; Since class B is declared outside of class A, the forward declaration must also be outside of class A. But the second misunderstanding is that an incomplete class (which is what B is between the forward declaration and the actual declaration) is only good for a very few things, for instance declaring pointers. In you code the compiler needs to know the size of B in order to generate code for A, but it does not know the size of B from just the forward declaration. The obvious solution to this is just to reverse the order in which you define your classes. The other solution would be to rewrite A like this class B; class A { B* b; }; By using a pointer you would not need to include the definition of B in the header file at all. This might also help with your other requirement that B not be usable by itself. Any suggestions? Am I missing something obvious ?Not obvious, but I''d guess you come from a Java background and haven''t quite absorbed yet that objects in C++ are values not references. Easily the most fundamental difference between Java and C++. john
> The correct way to make the forward declaration is class B; class A { B b; }; class B { }; Well Even that wont work ;( i copy pasted the above stuff and tried to compile it with g++ 3.3.2 on my Debian box. That thing apart, Since class B is declared outside of class A, the forward declaration must also be outside of class A. As per my previous posting... 1. anybody including "the .h" .... "which i didn''t intend to allow" ;). I dont want anybody use class B directly, i always want others to use it as A::B ,i.e as a part of class A. declaring in the way you suggested will allow everyone else to use it as is. making it a member would definitely impose using object of class B inside A''s object with this syntax. however, we can''t stop ppl from using bare class B objects. But the second misunderstanding is that an incomplete class (which is what B is between the forward declaration and the actual declaration) is only good for a very few things, for instance declaring pointers. Yes; pointers work as they have same sizes and we dont allocate memory. i tried that b4 posting here atfirst and it works fine. that is why i got confused about forward referencing. if the type is known, why can''t the compiler do a second pass to allocate the mem ? the compiler needs to know the size of B in order to generate code for A, but it does not know the size of B from just the forward declaration. that is why i asked: Am I not allowed to use forward referencing ? What exactly does the statement class B; in definition of A do? Doesn''t it introduce the typename in A? i mean doesn''t the compiler do a second pass to get the size and stuff?? The obvious solution to this is just to reverse the order in which you define your classes. hmmmm ..... but still if i define class B outside class A i will get it exposed to anyone including the header file. The other solution would be to rewrite A like this class B; class A { B* b; }; By using a pointer you would not need to include the definition of B in the header file at all. This might also help with your other requirement that B not be usable by itself. I dont know how by just declaring a pointer in a class can i restrict B''s usage. could you ellaborate please ? I did try and implement this with private constructors and friend functions. However that necessarilly means something else semantically. i want to implement something like-> A::B always. Any suggestions? Am I missing something obvious ?Not obvious
Thnx. cause i was getting pretty worried :) john
Thnx. ~M
Probably ''pimpl'' idiom can help you: www.gotw.ca/publications/mill04.htm Bye Fabio更多推荐
组合关系/前向参考问题。
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