[英]When and how should I use std::predicate
I am trying to constrain a Callable to return a boolean when evaluated.我试图约束 Callable 在评估时返回布尔值。 I have been trying to use the concept
std::predicate
, but it does not seem to do what I want it to do.我一直在尝试使用概念
std::predicate
,但它似乎没有做我想做的事。
So I defined my own concept, that is invokable and returns something convertible to a boolean.所以我定义了自己的概念,即可调用并返回可转换为布尔值的东西。 But again, I struggle understanding what I can or can not do with it, and I wonder what are the actual use cases of
std::predicate
?但同样,我很难理解我能用它做什么或不能做什么,我想知道
std::predicate
的实际用例是什么?
#include<concepts>
#include<string>
template<class F, class... Args>
concept Predicate = std::invocable<F, Args...> &&
std::convertible_to<std::invoke_result_t<F, Args...>, bool>;
int main(int argc, char *argv[])
{
constexpr Predicate auto f1 = [](){return true;}; // ok
constexpr std::predicate auto f2 = [](){return true;}; // ok
constexpr int x = 34;
constexpr Predicate auto f3 = [x](){ return x==42;}; // ok
// Pas ok: error: deduced initializer does not satisfy placeholder constraints
//constexpr Predicate auto f4 = [](auto x){ return x==42;};
//constexpr std::predicate auto f5 = [](auto x){ return x==42;};
}
You cannot have a function that takes a "thing that can be called".你不能有一个接受“可以调用的东西”的函数。 It must be a "thing that can be called with some set of arguments of known (at the time of the declaration) types".
它必须是“可以用一组已知(在声明时)类型的参数调用的东西”。 That's why
std::predicate
takes a set of arguments in addition to the potential callable type.这就是为什么
std::predicate
除了潜在的可调用类型之外还需要一组参数的原因。
Your first examples work because you didn't give the predicate concept any arguments and your functions also didn't take any parameters.您的第一个示例有效,因为您没有为谓词概念提供任何参数,并且您的函数也没有采用任何参数。 So an empty argument list matches the empty parameter list.
所以空参数列表匹配空参数列表。
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