为什么这个C++模板代码没有编译？

Question

有谁知道为什么这不会编译？我已经尝试了VS 2008和GCC 4.something并且都吐出了错误.无论我是否引用"ThisFunctionDoesNotCompile()"都无关紧要.

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会出现这个错误.

#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;
}

这些是我在VS 2008中遇到的错误:

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'

这些是GCC给我的:

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>'

Answer 1

在模板化类Base被实例化为Derived类的父类时,Derived类不是完整类型.

由于Base<Derived<DataClass> >是父类Derived<DataClass>,因此必须在实例化之前Derived<DataClass>对其进行实例化.因此,当Base<Derived<DataClass> >从模板构建类时,Derived<DataClass>行为就好像它是一个前向声明.正如您可能知道的那样,您不能引用不完整类型的成员,也不能引用前向声明的嵌套类型,因此您在这里运气不佳.

顺便说一句,这就是为什么使用模板实现正确协变的clone()方法很困难的原因.看到这里和这里(我的).