在GCC 8上运行的Constexpr计数器，不限于名称空间范围 [英] Constexpr counter that works on GCC 8, and is not restricted to namespace scope
问题描述
我正在尝试学习一些不可思议的状态模板元编程技巧。
（这就是我要学习的原因。不幸的是，该库在GCC 8和Clang上均不起作用。）
I'm trying to learn some arcane stateful template metaprogramming tricks.
(Here's why I want to learn it. Unfortunately this library doesn't work on GCC 8 nor on Clang.)
我首先需要的是 constexpr 计数器：
The first obvious thing I need is a constexpr counter:
/*something*/ constexpr int foo() /*something*/
int main()
{
constexpr int a = foo();
constexpr int b = foo();
constexpr int c = foo();
static_assert(a == 0 && b == 1 && c == 2);
}
最好是一个 tagged 计数器，因此我可以同时拥有多个计数器：
Preferably it should be a tagged counter, so that I can have several counters at the same time:
/*something*/ constexpr int foo() /*something*/
struct TagA {};
struct TagB {};
int main()
{
constexpr int a = foo<TagA>();
constexpr int b = foo<TagA>();
constexpr int c = foo<TagA>();
constexpr int d = foo<TagB>();
constexpr int e = foo<TagB>();
constexpr int f = foo<TagB>();
static_assert(a == 0 && b == 1 && c == 2);
static_assert(d == 0 && e == 1 && f == 2);
}
我做了一些研究，但可惜的是，我发现没有与之合作的计数器GCC 8。
I did some research, but alas, none of the counters I found worked with GCC 8.
- https： //github.com/DaemonSnake/unconstexpr -适用于GCC 7，不适用于GCC 8或Clang6。（在线尝试。）
- http://b.atch.se/posts/constexpr-counter/ -再次适用于GCC 7，不适用于GCC 8或Clang6。（在线尝试。）
- https://github.com/DaemonSnake/unconstexpr - Works with GCC 7, doesn't work with GCC 8 nor with Clang 6. (Try it online.)
- http://b.atch.se/posts/constexpr-counter/ - Once again, works with GCC 7 and doesn't work with GCC 8 nor with Clang 6. (Try it online.)
我还在这里找到了一些实现： C ++是否支持编译时计数器？，但是其中大多数限制在名称空间范围内，而其他限制再次与GCC 8不兼容。
I also found some implementations here: Does C++ support compile-time counters?, but most of them are limited to namespace scope, and others once again don't work with GCC 8.
我没有发现的是一个简单的概念验证可设置的constexpr标志： http://b.atch.se/posts/non-constant-constant-expressions/
What I did find was a simple proof-of-concept settable constexpr flag: http://b.atch.se/posts/non-constant-constant-expressions/
/*something*/ constexpr bool foo() /*something*/
constexpr bool a = foo();
constexpr bool b = foo();
constexpr bool c = foo();
static_assert (a == 0 && b == 1 && c == 1);
这个没有被标记，即每个翻译单元只能有一个，这不好。
This one is not tagged, i.e. you can only have one per translation unit, which is not good.
我已经设法基于它编写了自己的加标签实现：
I've managed to write my own tagged implementation based on it:
用法：
int main()
{
constexpr int c0_false = Meta::Flag<TagA>::ReadSet();
constexpr int c0_true = Meta::Flag<TagA>::ReadSet(); // Will continue to return true after this point.
static_assert(c0_false == 0);
static_assert(c0_true == 1);
constexpr int c1_false = Meta::Flag<TagB>::ReadSet();
constexpr int c1_true = Meta::Flag<TagB>::ReadSet(); // Will continue to return true after this point.
static_assert(c1_false == 0);
static_assert(c1_true == 1);
}
实施：
namespace Meta
{
template <typename T> class Flag
{
struct Dummy
{
constexpr Dummy() {}
friend constexpr void adl_flag(Dummy);
};
template <bool> struct Writer
{
friend constexpr void adl_flag(Dummy) {}
};
template <class Dummy, int = (adl_flag(Dummy{}),0)>
static constexpr bool Check(int)
{
return true;
}
template <class Dummy>
static constexpr bool Check(short)
{
return false;
}
public:
template <class Dummy = Dummy, bool Value = Check<Dummy>(0), int = sizeof(Writer<Value && 0>)>
static constexpr int ReadSet()
{
return Value;
}
template <class Dummy = Dummy, bool Value = Check<Dummy>(0)>
static constexpr int Read()
{
return Value;
}
};
}
接下来，我尝试进行实际计数。
Next, I tried to make an actual counter.
所需用法：
constexpr int c0 = Meta::TaggedCounter<TagA>::Value();
constexpr int c1 = Meta::TaggedCounter<TagA>::Value();
constexpr int c2 = Meta::TaggedCounter<TagA>::Value();
static_assert(c0 == 0);
static_assert(c1 == 1);
static_assert(c2 == 2);
我天真的尝试：（由于某种原因，它停止在 1 。）
My naïve attempt: (For some reason it stops at 1.)
namespace Meta
{
template <typename T> class TaggedCounter
{
template <int I> struct Tag {};
public:
template <int N = 0, bool B = Flag<Tag<N>>::ReadSet()> static constexpr int Value()
{
if constexpr (B)
return 1 + Value<N+1>();
else
return 0;
}
};
}
我该如何解决？
推荐答案
对于具有相同模板参数和相同参数的所有实例，constexpr函数模板的主体必须产生相同的答案。您需要添加一个间接级别，以便可以在依赖于第一个的模板参数的默认参数中进行计算。
The body of a constexpr function template must yield the same answer for all instatiations with the same template parameters and same arguments. You need to add a level of indirection, so the calculation can happen in the default argument of a template parameter dependent on the first.
请参见 https://gcc.godbolt.org/z/GHfKKf
namespace Meta
{
template <typename T,int I> struct Tag {};
template<typename T,int N,bool B>
struct Checker{
static constexpr int currentval() noexcept{
return N;
}
};
template<typename T,int N>
struct CheckerWrapper{
template<bool B=Flag<Tag<T,N>>::Read(),int M=Checker<T,N,B>::currentval()>
static constexpr int currentval(){
return M;
}
};
template<typename T,int N>
struct Checker<T,N,true>{
template<int M=CheckerWrapper<T,N+1>::currentval()>
static constexpr int currentval() noexcept{
return M;
}
};
template<typename T,int N,bool B=Flag<Tag<T,N>>::ReadSet()>
struct Next{
static constexpr int value() noexcept{
return N;
}
};
template <typename T> class TaggedCounter
{
public:
template <int N=CheckerWrapper<T,0>::currentval()> static constexpr int Value(){
return Next<T,N>::value();
}
};
}
这篇关于在GCC 8上运行的Constexpr计数器，不限于名称空间范围的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
