3

我有一个 Visual Studio 2008 C++ 应用程序,其中基类A_Base需要实例化其类型由父类定义的数据成员。例如:

template< typename T >
class A_Base
{
public: 
    typedef typename T::Foo Bar; // line 10

private:
    Bar bar_;
};

class A : public A_Base< A > 
{
public:
    typedef int Foo;
};

int _tmain( int argc, _TCHAR* argv[] )
{
    A a;
return 0;
}

不幸的是,编译器似乎不知道是什么T::Foo,直到为时已晚,我得到这样的错误:

1>MyApp.cpp(10) : error C2039: 'Foo' : is not a member of 'A'
1>        MyApp.cpp(13) : see declaration of 'A'
1>        MyApp.cpp(14) : see reference to class template instantiation 'A_Base<T>' being compiled
1>        with
1>        [
1>            T=A
1>        ]
1>MyApp.cpp(10) : error C2146: syntax error : missing ';' before identifier 'Bar'
1>MyApp.cpp(10) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>MyApp.cpp(10) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

有没有办法实现这种功能?

谢谢,保罗

4

3 回答 3

5

A_Base<A>A在尚未完成的地方实例化:

class A : public A_Base< A >

您可以考虑使用特征类:

template<class T> struct traits;

template< typename T >
class A_Base
{
public: 
    typedef typename traits<T>::Foo Bar; // line 10

private:
    Bar bar_;
};

class A; // Forward declare A

template<> // Specialize traits class for A
struct traits<A>
{
    typedef int Foo;
};

class A : public A_Base< A > {};

int main()
{
    A a;
}
于 2010-12-06T18:19:05.043 回答
3

您可以尝试以下方法:

template< typename T >
class A_Base
{
public: 
    typedef typename T::Foo Bar; // line 10

private:
    Bar bar_;
};

class A_Policy
{
public:
    typedef int Foo;
};

class A : public A_Base<A_Policy>
{};

int _tmain( int argc, _TCHAR* argv[] )
{
    A a;
return 0;
}
于 2010-12-06T18:14:14.797 回答
1

A依赖于类,而类A_Base又依赖于类A......等等。你在这里有一个递归。您需要Foo在单独的类中声明。

class A;

template<typename T> struct Foo;
template<> struct Foo<A> { typedef int type; };

template< typename T >
class A_Base
{
public: 
    typedef typename Foo<T>::type Bar; // line 10

private:
    Bar bar_;
};

class A : public A_Base< A > 
{
};

另见GotW #79

于 2010-12-06T18:20:21.057 回答