是否有理由使用std :: conjunction/std :: disjunction而不是在“&///||"上的折叠表达式? [英] Is there a reason to use std::conjunction/std::disjunction instead of a fold expression over "&&"/"||"?

问题描述

在某些特殊情况下，您不能正确地使用 std :: conjunction / std :: disjunction 而不使用更多的基本"(即语言功能而不是库功能)在&& / || 上折叠表达式?

Is there any specific cases you cannot correctly do with std::conjunction/std::disjunction and not using the more "fundamental" (i.e. language feature instead of library feature) fold expression over &&/||?

示例:

// func is enabled if all Ts... have the same type
template<typename T, typename... Ts>
std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...> >
func(T, Ts...) {
 // TODO something to show
}

vs

// func is enabled if all Ts... have the same type
template<typename T, typename... Ts>
std::enable_if_t<(std::is_same<T, Ts> &&...)>
func(T, Ts...) {
 // TODO something to show
}

使用fold表达式的版本更简短，通常更易读(尽管对此可能有不同的看法).因此，我不明白为什么将它与折叠表达式一起添加到库中.

The version using a fold expression is more brief and generally more readable (although opinions might differ on that). So I don't see why it was added to the library together with fold expressions.

推荐答案

std :: conjunction 短路了 :: value 实例化，而fold表达式没有.这意味着，鉴于:

std::conjunction short-circuits ::value instantiation, while the fold expression doesn't. This means that, given:

template <typename T> 
struct valid_except_void : std::false_type { };

template <> 
struct valid_except_void<void> { };

以下内容将编译:

template <typename... Ts>
constexpr auto test = std::conjunction_v<valid_except_void<Ts>...>;

constexpr auto inst = test<int, void>;

但是以下内容不会:

template <typename... Ts>
constexpr auto test = (valid_except_void<Ts>::value && ...);

constexpr auto inst = test<int, void>;

godbolt.org上的实时示例

来自 cppreference :

连接是短路的:如果存在带有 bool(Bi :: value)== false 的模板类型参数 Bi ，则实例化 conjunction<B1，...，BN> :: value 不需要实例化 j>的 Bj :: value .我.

Conjunction is short-circuiting: if there is a template type argument Bi with bool(Bi::value) == false, then instantiating conjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for j > i.

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