具有隐式参数的模板，前向声明，C ++ [英] Templates with implicit parameters, forward declaration, C++

问题描述

有一个带有隐式参数的模板类声明:

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> () {}
    ....
};

我试图在另一个头文件中使用顺延式声明:

I tried to use the fllowing forward declaration in a different header file:

Analysis.h

Analysis.h

template <typename T, const bool attribute = true>
class List;

但是G ++显示此错误:

But G++ shows this error:

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

If I use the forward declaration without implicit parameters

template <typename T, const bool attribute>
class List;

编译器不接受此构造

Analysis.h

Analysis.h

void function (List <Object> *list)
{
}

并显示以下错误(即不接受隐式值):

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

更新的问题:

我从模板定义中删除了默认参数:

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> () {}
    ....
};

使用类List的第一个文件具有带有参数属性隐式值的前向声明

The first file using class List has forward declaration with implicit value of the parameter attribute

Analysis1.h

Analysis1.h

template <typename T, const bool attribute = true>
class List;  //OK

class Analysis1
{
    void function(List <Object> *list); //OK
};

第二个使用List WITH类的类，使用隐式值进行正向定义

The second class using class List WITH forward definition using the implicit value

Analysis2.h

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
};

第二个使用类List的类，而没有使用隐式值进行正向定义

The second class using class List WITHOUT forward definition using the implicit value

Analysis2.h

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)
};

推荐答案

简单.从定义中删除默认值，因为您已经在前向声明中提到了该值.

Simple. Remove the default value from the definition, since you already mentioned that in the forward declaration.

template <typename Item, const bool attribute = true> //<--- remove this 'true`
class List: public OList <item, attribute>
{
  //..
};

写:

template <typename Item, const bool attribute>  //<--- this is correct!
class List: public OList <item, attribute>
{
  //..
};

在线演示: http://www.ideone.com/oj0jK

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