问题描述
因此，假设我有以下代码：
So, let's say I have this code:
template <class T1, class T2>
auto sum(T1 a, T2 b) ->decltype(a + b) {
return a + b;
}
template <class T1, class T2, class... T3>
auto sum(T1 a, T2 b, T3... tail) ->decltype(a + sum(b, tail...)) {
return a + sum(b, tail...);
}
我想调用函数 sum 以某种方式传递向量：
I would like to call function sum in a way I pass a vector:
vector<double> numbers = { 1, 2, 6, 5 };
应用作函数 sum 。我怎样才能做到这一点？在这种情况下，调用函数 sum 应该返回14。
that should be used as a list of arguments for function sum. How can I do that? Calling function sum should return 14 in this case.
推荐答案
std :: vector 是运行时的野兽。也就是说，它在堆上分配其缓冲区，并且通常在运行时允许任何操作。另一方面，可变参数模板 pearing在编译时完成。因此， std :: vector 和可变参数模板有些不相交。因此，不可能对向量进行任何操作。
std::vector is a run-time beast. That is, it allocates its buffer on the heap and generally any manipulation is allowed during run-time. On the other hand variadic template "pealing" is done during compile time. Consequently, a std::vector and variadic templates are somewhat "disjoint". Thus, it's not possible to do what you want with a vector.
如果要对向量的元素求和，可以使用<$在运行时完成c $ c> std :: accumulation ：
If you want to sum the elements of a vector this can be done in run-time using std::accumulate:
std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int sum = std::accumulate(v.begin(), v.end(), 0);
正如Brian在评论中提到的那样，您可以使用 std :: array 与 constexpr 函数结合使用进行编译时计算。下面显示了如何执行此操作的示例：
As Brian mentioned in the comments you could use a std::array for compile time computation in combination with constexpr functions. An example of how you could do this is displayed below:
namespace detail {
template <class T1, class T2>
constexpr auto sum_(T1 a, T2 b) {
return a + b;
}
template <class T1, class T2, class... T3>
constexpr auto sum_(T1 a, T2 b, T3... tail) {
return a + sum_(b, tail...);
}
template <typename T, std::size_t N, std::size_t... Is>
constexpr T sum_impl(std::array<T, N> const &src, std::index_sequence<Is...>) {
return sum_(src[Is]...);
}
}
template <typename T, std::size_t N>
constexpr T sum(std::array<T, N> const &arr) {
return detail::sum_impl(arr, std::make_index_sequence<N>{});
}
在上面的示例中，我标记了您的总和函数 constexpr 。您还可以弄清楚如何使用 std :: make_index_sequence 将数组的元素作为变量来填充 sum 函数。
In the above example I marked your sum functions constexpr. You can also figure out how you can use std::make_index_sequence to feed the elements of your array as arguments to your variadic sum function.
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