如何用元组模拟 std::array<15,int &>

Question

显然std::array<type,count>不能存储引用。但是可以写

std::tuple<int &, int &, int &, int &, int &>

并得到你期望的。参见

#include <array>
#include <tuple>
#include <iostream>

int main(){
    int a = 10;
    int b = 20;
    int c = 20;

    // Does not compile
    //std::array<int&,3> x = {a,b,c};

    using TInt3 = std::tuple<int&,int&,int&>;
    TInt3 y = {a,b,c};

    std::cout << sizeof(TInt3) << std::endl;
    std::get<0>(y)=11;
    std::cout << "a " << a << std::endl;
}

其中a 11是输出。然而，长手写出来很乏味。如何生成类型（在 c++11 中）

TupleAllSame<type, count>

所以下面是等价的

TupleAllSame<int &, 2>   <--> std::tuple<int &, int &>
TupleAllSame<double, 4>  <--> std::tuple<double, double, double, double>
TupleAllSame<std::string &, 3>  <--> std::tuple<std::string &, std::string &, std::string &>

Answer 1

基于 C++11 implementation of std::index_sequence（我将其塞入 std14 命名空间）：

namespace impl {
    template <class T, class Idx>
    struct TupleAllSame;
    
    template <class T, std::size_t... Idx>
    struct TupleAllSame<T, std14::index_sequence<Idx...>> {
        template <class U, std::size_t>
        using depend = U;
    
        using type = std::tuple<depend<T, Idx>...>;
    };
}

template <class T, std::size_t Size>
using TupleAllSame = typename impl::TupleAllSame<T, typename std14::make_index_sequence<Size>::type>::type;

See it live on Wandbox

Answer 2

std::reference_wrapper 可以用作数组的元素。语法有时会有点复杂，但它有效：

#include <functional>
#include <array>
#include <iostream>

int main(){
    int a = 10;
    int b = 20;
    int c = 20;

    using rint = std::reference_wrapper<int>;
    std::array<rint,3> x = {a,b,c};

    std::cout << "a " << a << std::endl;
    x[0].get()=11;
    std::cout << "a " << a << std::endl;
}

Answer 3

没有index_sequence的解决方案基于递归 (Wandbox Live)

#include<iostream>
#include <tuple>
using namespace std;

template <int N, typename T, typename SeqWithArgs>
struct append;

template <int N, typename T, template <typename...> class Seq, typename... Args >
struct append<N, T, Seq<Args...> >
{
    using type = typename append<N - 1, T, Seq<T, Args...> >::type;
};

template <typename T, template<typename...> class Seq, typename... Args>
struct append<0, T, Seq<Args...> >
{
    using type = Seq<Args...>;
};

template <typename T, size_t Size>
using TupleAllSame = typename append<Size, T, std::tuple<> >::type;

int main()
{
    int a = 0, b = 1, c = 2;
    TupleAllSame<int&, 3> t = { a,b, c };

    get<0>(t) = 10;
    cout << "a = " << a << endl;

    return 0;
}

如何用元组模拟 std::array<15,int &>

How to simulate std::array<15,int &> with tuples

c++

tuples

template-meta-programming

c++11