使用 std::get、std::tuple_size、std::tuple_element 对元组的组件求和
我有一个自定义类，它有一个类似元组的接口.因为我希望我的代码尽可能通用，所以我认为将我的算法基于函数 std::get、std::tuple_size, std::tuple_element 所以你只需要专门化这些函数来使用我的算法.让我们将需要这些函数特化的概念称为 Tuple.
I've got a custom class that has a tuple-like interface. Because I want my code to be as generic as possible, I thought that it would be a good idea to base my algorithms on the functions std::get, std::tuple_size, std::tuple_element so you just have to specialize these functions to use my algorithms. Let's call the concept that requires these function specializations Tuple.
现在我正在尝试总结 Tuple 的组成部分.函数声明应该是这样的:
Now I am trying to sum up the components of a Tuple. The function declaration should be something like this:
template <class Tuple>
int sum_components(const Tuple& t);
我想这涉及到很多模板编程，但我就是不知道该怎么做.
I guess that there is a lot of template programming involved but I just can't figure out how to do it.
对于添加，我将只使用全局 + 运算符的重载.
For the addition I would just use an overload of the global + operator.
我正在使用 c++1z.
I am using c++1z.
推荐答案
这在 c++17.
template<class Tuple>
decltype(auto) sum_components(Tuple const& tuple) {
auto sum_them = [](auto const&... e)->decltype(auto) {
return (e+...);
};
return std::apply( sum_them, tuple );
};
或 (...+e) 表示相反的折叠方向.
or (...+e) for the opposite fold direction.
在以前的版本中，正确的方法是编写自己的apply，而不是编写定制的实现.当您的编译器更新时，您可以删除代码.
In previous versions, the right approach would be to write your own apply rather than writing a bespoke implementation. When your compiler updates, you can then delete code.
在 c++14，我可能会这样做:
In c++14, I might do this:
// namespace for utility code:
namespace utility {
template<std::size_t...Is>
auto index_over( std::index_sequence<Is...> ) {
return [](auto&&f)->decltype(auto){
return decltype(f)(f)( std::integral_constant<std::size_t,Is>{}... );
};
}
template<std::size_t N>
auto index_upto() {
return index_over( std::make_index_sequence<N>{} );
}
}
// namespace for semantic-equivalent replacements of `std` code:
namespace notstd {
template<class F, class Tuple>
decltype(auto) apply( F&& f, Tuple&& tuple ) {
using dTuple = std::decay_t<Tuple>;
auto index = ::utility::index_upto< std::tuple_size<dTuple>{} >();
return index( [&](auto...Is)->decltype(auto){
auto target=std::ref(f);
return target( std::get<Is>( std::forward<Tuple>(tuple) )... );
} );
}
}
这非常接近 std::applyc++14'" rel="tag">c++14.(我滥用 std::ref 来获取 INVOKE 语义).(它不能完美地与右值调用程序一起工作，但这是非常极端的情况).
which is pretty close to std::apply in c++14. (I abuse std::ref to get INVOKE semantics). (It does not work perfectly with rvalue invokers, but that is very corner case).
在 c++11，我建议此时升级你的编译器.在 c++03 我建议此时升级你的工作.
In c++11, I would advise upgrading your compiler at this point. In c++03 I'd advise upgrading your job at this point.
以上所有方法都进行左右折叠.在某些情况下，二叉树折叠可能会更好.这更棘手.
All of the above do right or left folds. In some cases, a binary tree fold might be better. This is trickier.
如果您的 + 使用表达式模板，则上述代码由于生命周期问题而无法正常工作.在某些情况下，您可能必须为之后，强制转换"添加另一种模板类型以使临时表达式树进行评估.
If your + does expression templates, the above code won't work well due to lifetime issues. You may have to add another template type for "afterwards, cast-to" to cause the temporary expression tree to evaluate in some cases.
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