constexpr，数组和初始化 [英] constexpr, arrays and initialization

问题描述

在C ++世界中有什么可以让我想做的事情吗？

Is there anything in the world of C++ that would make what I'm trying to do possible?

template < typename T
         , size_t Size >
struct array
{
    constexpr T buf[Size];

    constexpr size_t size() const { return Size; }
};

template < typename T
         , size_t Size >
constexpr array<T,Size+1> push_back(array<T,Size> const& arr, T const& val)
{
    array<T,Size+1> arr_out = {{arr.buf, val}};

    return arr_out;
}

我要做的是创建一个用数据初始化的新数组在另一个，并把一个新的元素在结束。

What I'm trying to do is create a new array initialized with the data in the other, and put a new element on the end.

减少constexpr我可以得到它的工作循环初始化在push_back函数。看来你不能在constexpr函数，这是有一些意义，虽然我认为一个聪明的编译器可以解决这个问题。

Minus the constexpr I can get it to work by loop initializing in the push_back function. It appears you can't do that in constexpr functions, which makes some sense though I think a smart enough compiler could figure that out.

我确定它可以

推荐答案

指数招数，yay〜

template < typename T
         , size_t Size >
struct array
{
    T buf[Size]; // non-static data members can't be constexpr

    constexpr size_t size() const { return Size; }
};

namespace detail{
template< typename T, size_t N, size_t... Is>
constexpr array<T, N+1> push_back(array<T, N> const& arr, T const& val, indices<Is...>)
{
    // can only do single return statement in constexpr
    return {{arr.buf[Is]..., val}};
}
} // detail::

template < typename T, size_t Size >
constexpr array<T,Size+1> push_back(array<T,Size> const& arr, T const& val)
{
    return detail::push_back(arr, val, build_indices<Size>{});
}

Live example

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