具有隐式参数、前向声明、C++ 的模板
有一个带有隐式参数的模板类声明：
List.h
template <typename Item, const bool attribute = true>
class List: public OList <item, attribute>
{
public:
List() : OList<Item, attribute> () {}
....
};
我尝试在不同的头文件中使用以下前向声明：
Analysis.h
template <typename T, const bool attribute = true>
class List;
但 G++ 显示此错误：
List.h:28: error: redefinition of default argument for `bool attribute'
Analysis.h:43: error: original definition appeared here
如果我使用不带隐式参数的前向声明，
template <typename T, const bool attribute>
class List;
编译器不接受此构造
Analysis.h
void function (List <Object> *list)
{
}
并显示以下错误（即不接受隐式值）：
Analysis.h:55: error: wrong number of template arguments (1, should be 2)
Analysis.h:44: error: provided for `template<class T, bool destructable> struct List'
Analysis.h:55: error: ISO C++ forbids declaration of `list' with no type
更新的问题：
我从模板定义中删除了默认参数：
List.h
template <typename Item, const bool attribute>
class List: public OList <item, attribute>
{
public:
List() : OList<Item, attribute> () {}
....
};
使用类 List 的第一个文件有 使用参数属性隐式值的前向声明
Analysis1.h
template <typename T, const bool attribute = true>
class List; //OK
class Analysis1
{
void function(List <Object> *list); //OK
};
使用类 List 的第二个类，使用隐式值
Analysis2.h
template <typename T, const bool attribute = true> // Redefinition of default argument for `bool attribute'
class List;
class Analysis2
{
void function(List <Object> *list); //OK
};
进行前向定义使用类 List 的第二个类，不使用隐式值
Analysis2.h进行前向定义
template <typename T, const bool attribute> // OK
class List;
class Analysis2
{
void function(List <Object> *list); //Wrong number of template arguments (1, should be 2)
};
There is a declaration of template class with implicit parameters:
List.h
template <typename Item, const bool attribute = true>
class List: public OList <item, attribute>
{
public:
List() : OList<Item, attribute> () {}
....
};
I tried to use the fllowing forward declaration in a different header file:
Analysis.h
template <typename T, const bool attribute = true>
class List;
But G++ shows this error:
List.h:28: error: redefinition of default argument for `bool attribute'
Analysis.h:43: error: original definition appeared here
If I use the forward declaration without implicit parameters
template <typename T, const bool attribute>
class List;
compiler does not accept this construction
Analysis.h
void function (List <Object> *list)
{
}
and shows the following error (i.e. does not accept the implicit value):
Analysis.h:55: error: wrong number of template arguments (1, should be 2)
Analysis.h:44: error: provided for `template<class T, bool destructable> struct List'
Analysis.h:55: error: ISO C++ forbids declaration of `list' with no type
Updated question:
I removed the default parameter from the template definition:
List.h
template <typename Item, const bool attribute>
class List: public OList <item, attribute>
{
public:
List() : OList<Item, attribute> () {}
....
};
The first file using class List has forward declaration with implicit value of the parameter attribute
Analysis1.h
template <typename T, const bool attribute = true>
class List; //OK
class Analysis1
{
void function(List <Object> *list); //OK
};
The second class using class List WITH forward definition using the implicit value
Analysis2.h
template <typename T, const bool attribute = true> // Redefinition of default argument for `bool attribute'
class List;
class Analysis2
{
void function(List <Object> *list); //OK
};
The second class using class List WITHOUT forward definition using the implicit value
Analysis2.h
template <typename T, const bool attribute> // OK
class List;
class Analysis2
{
void function(List <Object> *list); //Wrong number of template arguments (1, should be 2)
};
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评论（5）
简单的。从定义中删除默认值，因为您已经在前向声明中提到了这一点。
编写：
在线演示：http://www.ideone.com/oj0jK
Simple. Remove the default value from the definition, since you already mentioned that in the forward declaration.
Write:
Online Demo : http://www.ideone.com/oj0jK
一个可能的解决方案是声明另一个头文件 List_fwd.h，
因此在 List.h 和 Analysis.h 中，您都在开头包含 List_fwd.h。所以List.h变成
And Analysis.h
A possible solution is to declare an other header file, List_fwd.h
So in both List.h and Analysis.h you include List_fwd.h at the beginning. So List.h becomes
And Analysis.h
您必须确保只有第一个声明具有参数的默认值。这可以通过首先定义仅前向声明标头，然后将其包含在
List.h和Analysis.h中来实现。在List.h的定义中，不要包含默认值。You must make sure only the first declaration has the default value of the parameter. This can be accomplished by first defining a forward-declaration-only header, then including it from both
List.handAnalysis.h. In the definition inList.h, do not include the default value.您只能在一处定义默认参数（对于给定的翻译）。在类声明中这样做是最有用的，在前面定义它是个坏主意。
转发不需要默认参数（在某些情况下您只需键入它）。
如果您确实想要一个默认参数，您可以创建另一个简单的模板类型，它与转发结合实现此功能。然后你可以通过 typedef 访问结果。您可以使用列表转发来做到这一点。
you may define the default parameter in only one place (for a given translation). it's most useful to do so at the class declaration, and a bad idea to define it at the forward.
a forward does not need the default parameter (you'll just have to type it in some cases).
you could create another simple template type which implements this in conjunction with a forward, if you really want a default parameter. then you access the result via a typedef. you can do this using a forward of List.
您必须将
List.h包含在您使用它的每个文件中。声明仅适用于非模板类型。对于模板类型，您必须包含每个编译单元的头文件。You must include
List.hin every file, where you are using it. Declaration is enought only for nontemplate types. For template types you must include header file for every compilation unit.