c++ - 仅专门化嵌套模板

Question

我有以下模板:

template<typename FirstParam>
struct First
{
    template<typename SecondParam>
    struct Second;
};

特化示例:

template<typename T> class D {};

template<>
template<>
struct First<C1>::Second<C1::C2>
{
    typedef D<C1::C2> type;
};

这是两个类同时特化的情况。但是有可能只专二等吗？

类似于:

template<typename OuterParam>
template<>
struct Outer<OuterParam>::Inner<typename OuterParam::C2>
{
    typedef E<typename OuterParam::C2> type;
};

(是的，我还需要第二个参数成为第一个参数的内部类。)

Answer 1

没有。 §14.7.3 [temp.expl.spec]/p16，重点补充:

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well.

您可以使用“部分”特化和 std::is_same 代替:

template<typename FirstParam>
struct First
{
    template<typename SecondParam, bool = std::is_same<typename FirstParam::C2,
                                                       SecondParam>::value>
    struct Second;

    template<class T>
    struct Second<T, true> { 
    };
};