c++11：从模板函数构建 std::tuple

Question

我有以下功能：

template<class T>
T Check(int index);

如何编写函数 CheckTuple，给定元组类型，通过调用 Check?[=16 来填充元组=]

例如：

CheckTuple< std::tuple<int, float, std::string> >()

将return以下元组：

std::make_tuple( Check<int>(1), Check<float>(2), Check<std::string>(3) )

我看到的其他问题涉及解包一个给定的元组，而不是用这种方式构建一个元组。

Answer 1

这是我的工作测试实现。（也许有人知道如何在简洁性方面改进它。我能以某种方式摆脱 TupleInfo 吗？）

#include <typeinfo>
#include <tuple>
#include <iostream>

template<class T>
T Check(int i) {
    std::cout << "getting a " << typeid(T).name() << " at position " << i << std::endl;
    return T();
}

template<typename Signature>
struct TupleInfo;

template<class T, class... Args>
struct TupleInfo< std::tuple<T, Args...> > {
    using Head = T;
    using Tail = std::tuple<Args...>;
};

template<int N, class Tuple>
struct TupleChecker {

    static Tuple CheckTuple() {
        auto t = std::make_tuple(Check<typename TupleInfo<Tuple>::Head>(N));
        return std::tuple_cat(t, TupleChecker<N+1, typename TupleInfo<Tuple>::Tail >::CheckTuple());
    }

};

template<int N>
struct TupleChecker<N, std::tuple<> > {

    static std::tuple<> CheckTuple() {
        return std::tuple<>();
    }

};

template<class Tuple>
Tuple CheckTuple() {
    return TupleChecker<1, Tuple>::CheckTuple();
}

int main() {

    std::tuple<> t0 = CheckTuple<std::tuple<> >();

    std::tuple<int> t1 = CheckTuple<std::tuple<int> >();

    std::tuple<int, float, std::string> t2 = CheckTuple<std::tuple<int, float, std::string> >();

    return 0;

}

Answer 2

使用 C++14 integer_sequence. If you don't have that available, here's a C++11 implementation written by Jonathan Wakely.

实现您正在寻找的东西变得非常简单

template<typename Tuple, int... I>
Tuple CallCheck(std::integer_sequence<int, I...>)
{
    return std::make_tuple(Check<typename std::tuple_element<I, Tuple>::type>(I)...);
}

template<typename Tuple>
Tuple CheckTuple()
{
    return CallCheck<Tuple>(std::make_integer_sequence<int, std::tuple_size<Tuple>::value>());
}

// Use it as 
auto tup = CheckTuple<std::tuple<int, float, std::string>>();

Live demo

c++11：从模板函数构建 std::tuple

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