在特定情况下,使用C++模板将整数映射到类型失败

Question

我试图在VC++ 2005中编译以下基于模板的代码.

    #include <iostream>
using namespace std;


/*
 * T is a template which maps an integer to a specific type.
 * The mapping happens through partial template specialization.
 * In the following T<1> is mapped to char, T<2> is mapped to long
 * and T<3> is mapped to float using partial template specializations
 */
template <int x>
struct T
{
public:
};

template<>
struct T<1>
{
public:
    typedef char xType;
};


template<>
struct T<2>
{
public:
    typedef long xType;
};

template<>
struct T<3>
{
public:
    typedef float xType;
};

// We can easily access the specific xType for a specific T<N>
typedef T<3>::xType x3Type;

/*!
 * In the following we are attempting to use T<N> inside another
 * template class T2<R>
 */

template<int r>
struct T2
{
    //We can map T<r> to some other type T3
    typedef T<r> T3;
    // The following line fails
    typedef T3::xType xType;
};

int main()
{
    T<1>::xType a1;
    cout << typeid(a1).name() << endl;
    T<2>::xType a2;
    cout << typeid(a2).name() << endl;
    T<3>::xType a3;
    cout << typeid(a3).name() << endl;
    return 0;
}

代码中有一个特殊的行不能编译:

typedef T3::xType xType;

如果我删除这一行,编译就可以了,结果是:

char
long
float

如果我保留此行,则会观察到编译错误.

main.cpp(53) : warning C4346: 'T<x>::xType' : dependent name is not a type
    prefix with 'typename' to indicate a type
    main.cpp(54) : see reference to class template instantiation 'T2<r>' being compiled
main.cpp(53) : error C2146: syntax error : missing ';' before identifier 'xType'
main.cpp(53) : error C4430: missing type specifier - int assumed. Note: C++ does not    support default-int

我无法弄清楚如何确保T :: xType可以被视为T2模板中的类型.任何帮助都非常感谢.

Answer 1

由于T3在模板类中依赖于模板参数,因此编译器无法确定T3::xType将引用什么(可能取决于r每个实例化中的实际类型T2<r>).

要告诉编译器T3::xType将是一个类型,您需要添加typename关键字:

typedef typename T3::xType xType;