为什么不编译这个C++模板代码？

有谁知道为什么这不会编译？我已经尝试了VS 2008和GCC 4.something，都吐出了错误。无论我是否引用“ThisFunctionDoesNotCompile（）”都没关系。为什么不编译这个C++模板代码？

我可以通过将'InternalType'作为第二个模板参数传递给Base来解决这个问题，但我仍然很好奇为什么这会出现错误。

#include <iostream> 
using namespace std; 

class DataClass 
{ 
public: 
    int m_data; 
}; 

template<typename DerivedType> 
class Base 
{ 
public: 
    int ThisFunctionCompiles() 
    { 
     // No problems here. 

     typename DerivedType::InternalType temp; 
     temp.m_data = 5; 
     return temp.m_data; 
    } 

    // error C2039: 'InternalType' : is not a member of 'Derived<InInternalType>' 
    typename DerivedType::InternalType ThisFunctionDoesNotCompile() 
    { 
     return static_cast<DerivedType*>(this)->GetInternalData(); 
    } 
}; 

template<typename InInternalType> 
class Derived : public Base<Derived<InInternalType> > 
{ 
public: 
    typedef InInternalType InternalType; 

    InternalType GetInternalData() 
    { 
     return m_internalData; 
    } 

private: 
    InternalType m_internalData; 


public: 
    void SetInternalData(int newVal) 
    { 
     m_internalData.m_data = newVal; 
    } 
}; 

int main() 
{ 

    Derived<DataClass> testDerived; 
    testDerived.SetInternalData(3); 

    cout << testDerived.GetInternalData().m_data << endl; 
    cout << testDerived.ThisFunctionCompiles() << endl; 

    // The compiler gives an error regardless of whether or not this is commented out. 
    //cout << testDerived.ThisFunctionDoesNotCompile().m_data << endl; 

    return 0; 
}

这是我得到的在VS 2008中的错误：

1>e:\test\generaltestprogram\generaltestprogram\main.cpp(27) : error C2039: 'InternalType' : is not a member of 'Derived<InInternalType>' 
1>  with 
1>  [ 
1>   InInternalType=DataClass 
1>  ] 
1>  e:\test\generaltestprogram\generaltestprogram\main.cpp(35) : see reference to class template instantiation 'Base<DerivedType>' being compiled 
1>  with 
1>  [ 
1>   DerivedType=Derived<DataClass> 
1>  ] 
1>  e:\test\generaltestprogram\generaltestprogram\main.cpp(58) : see reference to class template instantiation 'Derived<InInternalType>' being compiled 
1>  with 
1>  [ 
1>   InInternalType=DataClass 
1>  ] 
1>e:\test\generaltestprogram\generaltestprogram\main.cpp(27) : error C2146: syntax error : missing ';' before identifier 'ThisFunctionDoesNotCompile' 
1>e:\test\generaltestprogram\generaltestprogram\main.cpp(27) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int 
1>e:\test\generaltestprogram\generaltestprogram\main.cpp(28) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int 
1>e:\test\generaltestprogram\generaltestprogram\main.cpp(28) : warning C4183: 'ThisFunctionDoesNotCompile': missing return type; assumed to be a member function returning 'int'

而且这些都是海湾合作委员会给了我：

main.cpp: In instantiation of 'Base<Derived<DataClass> >': 
main.cpp:96: instantiated from 'Derived<DataClass>' 
main.cpp:119: instantiated from here 
main.cpp:88: error: no type named 'InternalType' in 'class Derived<DataClass>'

来源

2009-09-01 Screndib

在该模板类基本被实例化的时间派生类的父类，派生类不是完整类型。

由于Base<Derived<DataClass> >是Derived<DataClass>的父级，所以在Derived<DataClass>可以实例化之前必须实例化它。所以当Base<Derived<DataClass> >是从模板构建的时候，Derived<DataClass>的行为就好像它是一个前向声明。正如你可能知道的那样，你不能引用不完整类型的成员，也不能引用你的前向声明嵌套类型，所以你在这里运气不佳。

顺便说一句，这就是为什么使用模板很难实现一个正确的协变克隆（）方法。见here和here（我的）。

来源

2009-09-01 22:02:56

为什么不编译这个C++模板代码？

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