6

有谁知道为什么这不会编译？我已经尝试过 VS 2008 和 GCC 4.something 并且都吐出错误。我是否引用“ThisFunctionDoesNotCompile()”并不重要。

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>'
4

1 回答 1

12

在模板类 Base 被实例化为 Derived 类的父类时，Derived 类不是一个完整的类型。

由于Base<Derived<DataClass> >是 的父类Derived<DataClass>，所以必须先实例化才能Derived<DataClass>实例化。因此，当Base<Derived<DataClass> >从模板构建类时，Derived<DataClass>它的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，也不能前向声明嵌套类型，所以您在这里不走运。

顺便说一句，这就是为什么很难使用模板实现正确协变的 clone() 方法的原因。见这里和这里（我的）。

于 2009-09-01T22:02:56.843 回答