我试图为值列表中的值创建索引元查找器。
这是代码:
#include<type_traits>
#include<utility>
template <auto... Values>
struct ValueTplList;
template <auto>
constexpr int MetaFindV(int ind)
{ // not found case
return -1;
}
template <auto NeedleV, auto V, auto... Vs>
constexpr int MetaFindV(int ind = 0)
{
if constexpr (std::is_same_v<decltype(NeedleV), decltype(V)>)
{
return NeedleV == V ? ind : MetaFindV<NeedleV, Vs...>(ind + 1);
}
else
{
return MetaFindV<NeedleV, Vs...>(ind + 1);
}
}
// main
template <auto, typename>
struct MetaIndexOfV;
// destructurer
template <auto V, template<auto...> class Tmpl, auto... Vs>
struct MetaIndexOfV<V, Tmpl<Vs...> >
: std::integral_constant< int, MetaFindV<V, Vs...>() >
{};
// template variable version of it:
template <auto V, typename TemplInst>
constexpr int metaFindV_v = MetaIndexOfV<V, TemplInst>::value;
// tests
static_assert(metaFindV_v< 0, ValueTplList<0> > == 0 );
static_assert(metaFindV_v< 5, ValueTplList<0> > == -1);
static_assert(metaFindV_v< 0, ValueTplList<nullptr, 0> > == 1);
static_assert(metaFindV_v< 2, ValueTplList<1, (long)2, 2, 3, 4> > == 2);
static_assert(metaFindV_v< -1, ValueTplList<-2, -1, 42> > == 1);
在这里找到行为: https://godbolt.org/z/ukwxpN
你会发现这与gcc 7
及以上,以及clang 5
及以上编译良好。
我想知道我是否真的通过MSVC强制要求这种仇恨的标准做错了。我现在不能自己看到它:'(
它说:
(30):错误C2672:'MetaFindV':找不到匹配的重载函数>(31):注意:参见正在编译的类模板实例化'MetaIndexOfV>'的引用 (30):错误C2975:'V':'MetaFindV'的模板参数无效,预期的编译时常量表达式(12):注意:参见'V'的声明 (30):错误C2977:'MetaFindV':模板参数太多
我认为这是抱怨这条线
: std::integral_constant< int, MetaFindV<V, Vs...>() >
就像MetaFindV<V, Vs...>()
没有匹配功能一样。
(最后注意:如果你为auto
更改所有typename
, - 稍微调整一下 - 整个事情开始起作用(但当然只对类型和类型列表而言),即使代码模式完全相同。)
不是答案(对不起:我怀疑这是一个MSVC错误,但我不确定)但是长篇评论。
我提出了一种完全不同的方式来获得你想要的东西:
#include <type_traits>
#include <string>
template <auto... Values>
struct ValueTplList
{ };
template <auto, auto>
struct strongSame : public std::false_type
{ };
template <auto A>
struct strongSame<A, A> : public std::true_type
{ };
template <auto TargetVal, auto ... Values>
constexpr int foo (ValueTplList<Values...> const &)
{
int ind = -1;
(void)( (++ind, strongSame<TargetVal, Values>::value)
|| ... || (++ind, true) );
return std::size_t(ind) == sizeof...(Values) ? -1 : ind;
}
template <auto V, typename TemplInst>
constexpr int metaFindV_v = foo<V>(TemplInst{});
// tests
static_assert(metaFindV_v< 0, ValueTplList<0> > == 0 );
static_assert(metaFindV_v< 5, ValueTplList<0> > == -1);
static_assert(metaFindV_v< 0, ValueTplList<nullptr, 0> > == 1);
static_assert(metaFindV_v< 2, ValueTplList<1, (long)2, 2, 3, 4> > == 2);
static_assert(metaFindV_v< -1, ValueTplList<-2, -1, 42> > == 1);
int main ()
{
}
我努力尝试失败了。这似乎是MSVC中的一个普通错误。
我感兴趣的人的解决方案是完全放弃:'(((相反,我选择了类型和值包装器,如下所示:
#include<type_traits>
#include<utility>
#include<variant>
template <typename... Types>
struct TypeList;
// auto: http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0127r1.html
template <auto... Values>
struct ValueTplList;
// meta technique to find index of a type in a typelist
template <typename>
constexpr int MetaFind(int ind)
{ // not found case
return -1;
}
template <typename NeedleT, typename T, typename... Ts>
constexpr int MetaFind(int ind = 0)
{
if (std::is_same_v<NeedleT, T>)
{
return ind;
}
else
{
return MetaFind<NeedleT, Ts...>(ind + 1);
}
}
// flat 2 template parameters versions
template <typename T, typename T2>
struct MetaIndexOf : std::integral_constant<int, -1>
{};
template <typename T>
struct MetaIndexOf<T,T> : std::integral_constant<int, 0>
{};
// destructurer version (access the contents of a typelist)
template <typename T, template<typename...> class Tmpl, typename... Ts>
struct MetaIndexOf<T, Tmpl<Ts...> >
: std::integral_constant< int, MetaFind<T, Ts...>() >
{};
// template variable version of it:
template <typename T, typename TemplInst>
constexpr int metaFind_v = MetaIndexOf<T, TemplInst>::value;
// test
static_assert(metaFind_v< bool, bool > == 0 );
static_assert(metaFind_v< bool, int > == -1 );
static_assert(metaFind_v< int, TypeList<bool, float, int, double> > == 2);
static_assert(metaFind_v< double, TypeList<bool, float, int, double> > == 3);
static_assert(metaFind_v< bool, TypeList<bool, float, int, double> > == 0);
static_assert(metaFind_v< long, TypeList<bool, float, int, double> > == -1);
// make a metaFind for values:
// let's try with a value wrapped in a type
template <auto V>
struct AnyConstantV : std::integral_constant<decltype(V), V>
{};
template <typename>
struct BoxAll;
// convenience helper to directly make a typelist of boxed values from values
template< template<auto...> class VL, auto... Vs >
struct BoxAll< VL<Vs...> >
{
using type = TypeList< AnyConstantV<Vs>... >;
};
static_assert( std::is_same_v< BoxAll<ValueTplList<0>>::type, TypeList<AnyConstantV<0>> > );
template <auto V, typename VL>
constexpr int metaFindV_v = metaFind_v< AnyConstantV<V>, typename BoxAll<VL>::type >;
// tests
static_assert(metaFindV_v< 0, ValueTplList<0> > == 0 );
static_assert(metaFindV_v< 5, ValueTplList<0> > == -1);
static_assert(metaFindV_v< 0, ValueTplList<nullptr, 0> > == 1);
// god dammnit visual studio ! ..... :(
static_assert(metaFindV_v< 2, ValueTplList<1, (long)2, 2, 3, 4> > == 2);
static_assert(metaFindV_v< -1, ValueTplList<-2, -1, 42> > == 1);
你可以在以下地方玩: https://godbolt.org/z/bnfZ5r
请注意,static_assert
与2失败是悲惨和错误的。再次,clang在这里通过罚款。最有趣的是,我不是在欺骗你,intellisense是正确的:(MSVC在这里建立没有断言,它应该在哪里)