如何检测可变参数模板中的第一个和最后一个参数? [英] How to detect the first and the last argument in the variadic templates?

问题描述

如何检测可变参数模板中的第一个和最后一个参数?

How to detect the first and the last argument in the variadic templates?

对于第一个参数很容易(只需将 sizeof...(T) 与 0 进行比较)，但是有没有办法检测最后一个元素?

For the 1st argument it is easy (just compare sizeof...(T) with 0), but is there a way to detect the last element?

例子:

#include <iostream>
#include <typeinfo>

template < class... T >
struct A
{
    int foo(int k){ return k; };
};

template < class T1, class... T >
struct A< T1, T... >
{
    A() :a()
    {
        std::cout<<"A  i="<<sizeof...(T)<<std::endl
                 <<"   a type = " << typeid(T1).name()<<std::endl;
    }

    int foo(int k){ return anotherA.foo( a.foo(k) ); };

    T1 a;
    A< T... > anotherA;
};

struct B1
{
    B1(){ std::cout<<"b1"<<std::endl; };
    int foo(int k){ std::cout<<"b1::foo() k="<<k<<std::endl; return k+1; };
};
struct B2
{
    B2(){ std::cout<<"b2"<<std::endl; };
    int foo(int k){ std::cout<<"b2::foo() k="<<k<<std::endl; return k+2; };
};
struct B3
{
    B3(){ std::cout<<"b3"<<std::endl; };
    int foo(int k){ std::cout<<"b3::foo() k="<<k<<std::endl; return k+3; };
};

int main ()
{
    A< B3, B2, B1 > a;

    std::cout<<"the value is "
             <<a.foo(5)
             << std::endl;
}

推荐答案

如果这是您想要的，我并不肯定.但这里有两个名为 first 和 last 的实用程序，它们分别采用可变参数模板和 typedef 第一个和最后一个类型:

I'm not positive if this is what you want. But here are two utilities named first and last that take variadic templates and typedef the first and last type respectively:

#include <iostream>
#include <typeinfo>

template <class T1, class ...T>
struct first
{
    typedef T1 type;
};

template <class T1, class ...T>
struct last
{
    typedef typename last<T...>::type type;
};

template <class T1>
struct last<T1>
{
    typedef T1 type;
};

template <class ...T>
struct A
{
    typedef typename first<T...>::type first;
    typedef typename last<T...>::type  last;
};

struct B1 {};
struct B2 {};
struct B3 {};

int main()
{
    typedef A<B1, B2, B3> T;
    std::cout << typeid(T::first).name() << '\n';
    std::cout << typeid(T::last).name() << '\n';
}

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