[英]How do I filter a tuple of types depending on whether an expression is valid in Boost.Hana?
In a reduced example I have two types with in a tuple and I want to create another tuple that only contains the types where an expression is valid (in this example I'm using the + operator). 在一个简化的示例中,我在一个元组中有两种类型,我想创建另一个仅包含表达式有效的类型的元组(在此示例中,我使用+运算符)。
My attempt is like so: 我的尝试是这样的:
#include <boost/hana.hpp>
#include <boost/hana/experimental/printable.hpp>
#include <iostream>
namespace hana = boost::hana;
struct foo {};
const auto result{hana::filter(hana::tuple_t<int, foo>, [](auto type) {
const auto has_plus{hana::is_valid([](auto t)
-> decltype((void) hana::traits::declval(t) + hana::traits::declval(t)) {})};
return has_plus(type);
})};
int main()
{
std::cout << hana::experimental::print(result) << std::endl;
}
This outputs '()', ie. 这输出'()',即。 no types matched where I would expect that the tuple
result
should contain type<int>
. 没有任何类型与我期望的元组
result
应包含type<int>
匹配。
I am using the Boost.Hana version that is packaged with Boost 1.62. 我正在使用Boost 1.62打包的Boost.Hana版本。
Your problem is with operator precedence. 您的问题在于运算符的优先级。
(void)hana::traits::declval(t) + hana::traits::declval(t)
is the same as 是相同的
((void)hana::traits::declval(t)) + hana::traits::declval(t)
The following yields the result you expected: 以下将产生您期望的结果:
(void)(hana::traits::declval(t) + hana::traits::declval(t))
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