[英]std::is_same doesn't work through decltype of constexpr auto variable
I was trying to static_assert
that some meta transformer algorithm worked, and it incredibly did not compare to same, even though the typeid().name()
returned the exact same string. 我正在尝试static_assert
一些元转换器算法工作,并且即使typeid().name()
返回了完全相同的字符串,它也难以与之相比。
A repetition of the type expression in the typedef could fix the is_same
, but I can't understand how repeating the initializer expression in the typedef changes the type, over taking the decltype of an auto variable initialized with that same expression . 在typedef中重复类型表达式可以修复is_same
,但是我不理解在typedef中重复初始化 is_same
函数表达式会如何改变类型,而不是使用用同一表达式初始化的auto变量的decltype 。
A more concrete explanation of what I was doing: 我正在做什么的更具体的解释:
I did a meta transformer that can transform a meta value list (containing enumerators) to a std::tuple of all enumerators. 我做了一个元转换器,可以将元值列表(包含枚举器)转换为所有枚举器的std :: tuple。 Then I checked that the tuple generated was the one I expected. 然后,我检查生成的元组是否是我所期望的。
all code hereunder, check the line with the comments // works // doesn't work 下面的所有代码,请在注释行中// //起作用//不起作用
And the final test is at the end. 最后的测试在最后。
#include <type_traits>
#include <tuple>
template<typename T> struct ValueListAsTuple; // master template
// destructurer of list:
template<template <auto...> class L, auto... Vs>
struct ValueListAsTuple<L<Vs...>>
{
static constexpr auto value = std::make_tuple(Vs...);
//using type = decltype(std::make_tuple(Vs...)); // works
using type = decltype(value); // doesn't work
};
// template typedef
template<typename T> using ValueListAsTuple_t = typename ValueListAsTuple<T>::type;
struct Kind
{
enum EnumType { E1, E2 };
template <auto... Values> struct MetaVals{}; // meta value list
using MetaValueList = MetaVals<
E1,
E2
>;
};
int main(int argc, const char* argv[])
{
auto tuple = ValueListAsTuple_t< Kind::MetaValueList > {};
//std::cout << typeid(tuple).name() << '\n';
// this prints: "class std::tuple<enum Kind::EnumType, enum Kind::EnumType>"
// manual re-creation of the type, for testing:
std::tuple< Kind::EnumType, Kind::EnumType > t2;
//std::cout << typeid(t2).name() << '\n';
// this prints the exact same thing.
static_assert( std::is_same_v< std::tuple_element<0, decltype(tuple)>::type, Kind::EnumType > );
static_assert( std::is_same_v< std::tuple_element<1, decltype(tuple)>::type, Kind::EnumType > );
// WHAT ???
static_assert( std::is_same_v<
ValueListAsTuple_t< Kind::MetaValueList >,
std::tuple< Kind::EnumType, Kind::EnumType >
> );
// is_convertible_v works but I don't care ! wth ?
}
So as you can see, is_same
could not deduce the same type when the type
was declared through value
but it worked if I repeat std::make_tuple(..
. I can't see where there is any difference between the 2 forms. 因此,大家可以看到, is_same
当不能推断出相同类型的type
通过声明value
,但它的工作,如果我重复std::make_tuple(..
我看不出哪里有2种形式之间的任何差别。
I even checked element by element that the types are the same at each index of the tuple. 我什至逐个元素地检查了元组的每个索引的类型是否相同。
Check that in action in godbolt: 检查一下Godbolt中的动作:
https://godbolt.org/z/QUCXMB https://godbolt.org/z/QUCXMB
The behavior is the same across gcc/clang/msvc 在gcc / clang / msvc中,行为是相同的
It's a trivial error, really, but hard to spot unfortunately. 确实,这是一个小错误,但不幸的是很难发现。 The issue you face is because constexpr
implies const
. 您面临的问题是因为constexpr
隐含const
。 So in your example type
is const tuple<...>
, which is not the same type as the non cv-qualified type you check. 因此,在示例中, type
为const tuple<...>
,它与您检查的非cv限定类型不同。 A short fix to the alias should make your test pass: 对别名的简短修正应使您的测试通过:
using type = std::remove_const_t<decltype(value)>;
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