I have inherited the following:
template <typename T>
concept IsAwaiter = requires {
typename T::await_ready;
typename T::await_suspend;
typename T::await_resume;
};
template <typename ...AWAITABLES>
concept IsAwaitables = typename std::conjunction<IsAwaiter<AWAITABLES>...>::type;
Building this with clang 10.0.0 results in the following error:
IsAwaiter.h:43:50: error: template argument for template type parameter must be a type
Perhaps just a simple syntax issue, but I've found it hard to find an example which shows how to create a concept based on a variadic template concept parameter.
Any help appreciated!
std::conjunction is for type traits. std::conjunction<IsAwaiter<AWAITABLES>...> is a type with a member static bool value = (IsAwaiter<AWAITABLES>::value && ...), where each IsAwaiter<AWAITABLES> is itself expected to be a type trait with its own static bool member value. This is nonsensical when IsAwaiter<AWAITABLES> is a concept, because concepts are not type traits. They are "just" booleans. Use a fold expression.
template <typename... AWAITABLES>
concept IsAwaitables = (IsAwaiter<AWAITABLES> && ...);
That's it.
struct Dummy {
using await_ready = Dummy;
using await_suspend = Dummy;
using await_resume = Dummy;
};
int main() {
static_assert(IsAwaitables<>);
static_assert(IsAwaitables<Dummy>);
static_assert(IsAwaitables<Dummy, Dummy>);
}
As HTNW points out, you want:
template <typename ...T>
concept IsAwaitables = (IsAwaiter<T> && ...);
But really, do you even need this concept at all? You can just use IsAwaiter directly. And it probably should just be named Awaiter - the typical convention for concepts is to name them as nouns rather than questions (e.g. Range vs IsRange).
If you're taking a parameter pack, you would want to use it anyway:
template <Awaiter... T>
void f(T... awaiters);
and the same with the abbreviated function template syntax:
void f(Awaiter auto... awaiters);
or if you have a fixed number of them, it especially doesn't make sense:
template <Awaiter T, Awaiter U>
void f(T, U);
And even in other contexts where it doesn't neatly fit, it seems better to just manually use Awaiter with a fold-expression. So I question the need for the conjunction concept to begin with.
Related
I'm not sure if this is possible, but I would like to count the number of template arguments of any class like:
template <typename T>
class MyTemplateClass { ... };
template <typename T, typename U>
class MyTemplateClass2 { ... };
such that template_size<MyTemplateClass>() == 1 and template_size<MyTemplateClass2>() == 2. I'm a beginner to template templates, so I came up with this function which of course does not work:
template <template <typename... Ts> class T>
constexpr size_t template_size() {
return sizeof...(Ts);
}
because Ts can not be referenced. I also know that it might come to problems when handling variantic templates, but that is not the case, at least for my application.
Thx in advance
There is one...
° Introduction
Like #Yakk pointed out in his comment to my other answer (without saying it explicitly), it is not possible to 'count' the number of parameters declared by a template. It is, on the other hand, possible to 'count' the number of arguments passed to an instantiated template.
Like my other answer shows it, it is rather easy to count these arguments.
So...
If one cannot count parameters...
How would it be possible to instantiate a template without knowing the number of arguments this template is suppose to receive ???
Note
If you wonder why the word instantiate(d) has been stricken throughout this post,
you'll find its explanation in the footnote. So keep reading... ;)
° Searching Process
If one can manage somehow to try to instantiate a template with an increasing number of arguments, and then, detect when it fails using SFINAE (Substitution Failure Is Not An Error), it should be possible to find a generic solution to this problem... Don't you think ?
Obviously, if one wants to be able to also manage non-type parameters, it's dead.
But for templates having only typename parameters...
There is one...
Here are the elements with which one should be able to make it possible:
A template class declared with only typename parameters can receive any type as argument. Indeed, although there can have specializations defined for specific types,
a primary template cannot enforce the type of its arguments.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The above statement might no longer be true from C++20 concepts.
I cannot try ATM, but #Yakk seems rather confident on the subject. After his comment:
I think concepts breaks this. "a primary template cannot enforce the type of its arguments." is false.
He might be right if constraints are apply before the template instantiation. But...
By doing a quick jump to the introduction to Constraints and concepts, one can read, after the first code example:
"Violations of constraints are detected at compile time, early in the template instantiation process, which leads to easy to follow error messages."
To be confirmed...
It is perfectly possible to create a template having for sole purpose to
be instantiated with any number of arguments. For our use case here, it might contain only ints... (let's call it IPack).
It is possible to define a member template of IPack to define the Next IPack by adding an int to the arguments of the current IPack. So that one can progressively increase its number of arguments...
Here is maybe the missing piece. It is maybe something that most people don't realize.
(I mean, most of us uses it a lot with templates when, for example, the template accesses a member that one of its arguments must have, or when a trait tests for the existence of a specific overload, etc...)
But I think it might help in finding solutions sometimes to view it differently and say:
It is possible to declare an arbitrary type, built by assembling other types, for which the evaluation by the compiler can be delayed until it is effectively used.
Thus, it will be possible to inject the arguments of an IPack into another template...
Lastly, one should be able to detect if the operation succeeded with a testing trait making use of decltype and std::declval. (note: In the end, none of both have been used)
° Building Blocks
Step 1: IPack
template<typename...Ts>
struct IPack {
private:
template<typename U> struct Add1 {};
template<typename...Us> struct Add1<IPack<Us...>> { using Type = IPack<Us..., int>; };
public:
using Next = typename Add1<IPack<Ts...>>::Type;
static constexpr std::size_t Size = sizeof...(Ts);
};
using IPack0 = IPack<>;
using IPack1 = typename IPack0::Next;
using IPack2 = typename IPack1::Next;
using IPack3 = typename IPack2::Next;
constexpr std::size_t tp3Size = IPack3::Size; // 3
Now, one has a means to increase the number of arguments,
with a convenient way to retrieve the size of the IPack.
Next, one needs something to build an arbitrary type
by injecting the arguments of the IPack into another template.
Step 2: IPackInjector
An example on how the arguments of a template can be injected into another template.
It uses a template specialization to extract the arguments of an IPack,
and then, inject them into the Target.
template<typename P, template <typename...> class Target>
struct IPackInjector { using Type = void; };
template<typename...Ts, template <typename...> class Target>
struct IPackInjector<IPack<Ts...>, Target> { using Type = Target<Ts...>; };
template<typename T, typename U>
struct Victim;
template<typename P, template <typename...> class Target>
using IPInj = IPackInjector<P, Target>;
//using V1 = typename IPInj<IPack1, Victim>::Type; // error: "Too few arguments"
using V2 = typename IPInj<IPack2, Victim>::Type; // Victim<int, int>
//using V3 = typename IPInj<IPack3, Victim>::Type; // error: "Too many arguments"
Now, one has a means to inject the arguments of an IPack
into a Victim template, but, as one can see, evaluating Type
directly generates an error if the number of arguments does not
match the declaration of the Victim template...
Note
Have you noticed that the Victim template is not fully defined ?
It is not a complete type. It's only a forward declaration of a template.
The template to be tested will not need to be a complete type
for this solution to work as expected... ;)
If one wants to be able to pass this arbitrary built type to some detection trait one will have to find a way to delay its evaluation.
It turns out that the 'trick' (if one could say) is rather simple.
It is related to dependant names. You know this annoying rule
that enforces you to add ::template everytime you access a member template
of a template... In fact, this rule also enforces the compiler not to
evaluate an expression containing dependant names until it is
effectively used...
Oh I see ! ...
So, one only needs to prepare the IPackInjectors without
accessing its Type member, and then, pass it to our test trait, right ?
It could be done using something like that:
using TPI1 = IPackInjector<IPack1, Victim>; // No error
using TPI2 = IPackInjector<IPack2, Victim>; // No error
using TPI3 = IPackInjector<IPack3, Victim>; // No error
Indeed, the above example does not generate errors, and it confirms
that there is a means to prepare the types to be built and evaluate
them at later time.
Unfortunately, it won't be possible to pass these pre-configured
type builders to our test trait because one wants to use SFINAE
to detect if the arbitrary type can be instantiated or not.
And this is, once again, related to dependent name...
The SFINAE rule can be exploited to make the compiler silently
select another template (or overload) only if the substitution
of a parameter in a template is a dependant name.
In clear: Only for a parameter of the current template instantiation.
Hence, for the detection to work properly without generating
errors, the arbitrary type used for the test will have to be
built within the test trait with, at least, one of its parameters.
The result of the test will be assigned to the Success member...
Step 3: TypeTestor
template<typename T, template <typename...> class C>
struct TypeTestor {};
template<typename...Ts, template <typename...> class C>
struct TypeTestor<IPack<Ts...>, C>
{
private:
template<template <typename...> class D, typename V = D<Ts...>>
static constexpr bool Test(int) { return true; }
template<template <typename...> class D>
static constexpr bool Test(...) { return false; }
public:
static constexpr bool Success = Test<C>(42);
};
Now, and finally, one needs a machinery that will successively try
to instantiate our Victim template with an increasing number of arguments. There are a few things to pay attention to:
A template cannot be declared with no parameters, but it can:
Have only a parameter pack, or,
Have all its parameters defaulted.
If the test procedure begins by a failure, it means that the template must take more arguments. So, the testing must continue until a success, and then, continue until the first failure.
I first thought that it might make the iteration algorithm using template specializations a bit complicated... But after having thought a little about it, it turns out that the start conditions are not relevant.
One only needs to detect when the last test was true and next test will be false.
There must have a limit to the number of tests.
Indeed, a template can have a parameter pack, and such a template can receive an undetermined number of arguments...
Step 4: TemplateArity
template<template <typename...> class C, std::size_t Limit = 32>
struct TemplateArity
{
private:
template<typename P> using TST = TypeTestor<P, C>;
template<std::size_t I, typename P, bool Last, bool Next>
struct CheckNext {
using PN = typename P::Next;
static constexpr std::size_t Count = CheckNext<I - 1, PN, TST<P>::Success, TST<PN>::Success>::Count;
};
template<typename P, bool Last, bool Next>
struct CheckNext<0, P, Last, Next> { static constexpr std::size_t Count = Limit; };
template<std::size_t I, typename P>
struct CheckNext<I, P, true, false> { static constexpr std::size_t Count = (P::Size - 1); };
public:
static constexpr std::size_t Max = Limit;
static constexpr std::size_t Value = CheckNext<Max, IPack<>, false, false>::Count;
};
template<typename T = int, typename U = short, typename V = long>
struct Defaulted;
template<typename T, typename...Ts>
struct ParamPack;
constexpr std::size_t size1 = TemplateArity<Victim>::Value; // 2
constexpr std::size_t size2 = TemplateArity<Defaulted>::Value; // 3
constexpr std::size_t size3 = TemplateArity<ParamPack>::Value; // 32 -> TemplateArity<ParamPack>::Max;
° Conclusion
In the end, the algorithm to solve the problem is not that much complicated...
After having found the 'tools' with which it would be possible to do it, it only was a matter, as very often, of putting the right pieces at the right places... :P
Enjoy !
° Important Footnote
Here is the reason why the word intantiate(d) has been stricken at the places where it was used in relation to the Victim template.
The word instantiate(d) is simply not the right word...
It would have been better to use try to declare, or to alias the type of a future instantiation of the Victim template.
(which would have been extremely boring) :P
Indeed, none of the Victim templates gets ever instantiated within the code of this solution...
As a proof, it should be enough to see that all tests, made in the code above, are made only on forward declarations of templates.
And if you're still in doubt...
using A = Victim<int>; // error: "Too few arguments"
using B = Victim<int, int>; // No errors
template struct Victim<int, int>;
// ^^^^^^^^^^^^^^^^
// Warning: "Explicit instantiation has no definition"
In the end, there's a full sentence of the introduction which might be stricken, because this solution seems to demonstrate that:
It is possible to 'count' the number of parameters declared by a template...
Without instantiation of this template.
#include <utility>
#include <iostream>
template<template<class...>class>
struct ztag_t {};
template <template<class>class T>
constexpr std::size_t template_size_helper(ztag_t<T>) {
return 1;
}
template <template<class, class>class T>
constexpr std::size_t template_size_helper(ztag_t<T>) {
return 2;
}
template <typename T>
class MyTemplateClass { };
template <typename T, typename U>
class MyTemplateClass2 { };
template<template<class...>class T>
struct template_size:
std::integral_constant<
std::size_t,
template_size_helper(ztag_t<T>{})
>
{};
int main() {
std::cout << template_size<MyTemplateClass>::value << "\n";
std::cout << template_size<MyTemplateClass2>::value << "\n";
}
I know of no way without writing out the N overloads to support up to N arguments.
Live example.
Reflection will, of course, make this trivial.
° Before Reading This Post
This post does not answer to "How to get the number of parameters",
it answers to "how to get the number of arguments"...
It is let here for two reasons:
It might help someone who would have mixed up (like I did)
the meaning of parameters and arguments.
The techniques used in this post are closely related to the ones used
to produce the correct answer I've posted as a separate answer.
See my other answer for finding "the number of parameters" of a template.
The answer of Elliott looks more like what one usually does (though the primary template should be fully defined and "do something" IMHO). It uses a template specialization for when a template is passed as argument to the primary template.
Meanwhile, the answer of Elliott vanished...
So I've posted something similar to what he showed below.
(see "Generic Solution")
But, just to show you that you weren't that far from a working solution, and, because I noticed that you have used a function for your try, and, you declared this function constexpr, you could have written it like that:
Note
It is a 'fancy solution', but it works...
template <typename T> class MyTemplateClass {};
template <typename T, typename U> class MyTemplateClass2 {};
template <template <typename...> class T, typename...Ts>
constexpr const size_t template_size(T<Ts...> && v)
{
return sizeof...(Ts);
}
// If the target templates are complete and default constructible.
constexpr size_t size1 = template_size(MyTemplateClass<int>{});
constexpr size_t size2 = template_size(MyTemplateClass2<int, short>{});
// If the target templates are complete but NOT default constructible.
constexpr size_t size3 = template_size(decltype(std::declval<MyTemplateClass<int>>()){});
constexpr size_t size4 = template_size(decltype(std::declval<MyTemplateClass2<int, short>>()){});
Explanation
You said "because Ts can not be referenced", which is true and false, because of the way you made the declaration of template_size.
That is, a template template parameter cannot declare parameters itself (where you placed Ts in the declaration of the function template). It is allowed to do so to give a clue of what the template argument is expected to receive as argument, but it is not a declaration of a parameter name for the current template declaration...
(I hope it's clear enough) ;)
Obviously, it might be a little bit over complicated, but it worth knowing I think that such a construct is possible also... ;)
Generic Solution
template <typename T> class MyTemplateClass {};
template <typename T, typename U> class MyTemplateClass2 {};
template<typename T>
struct NbParams { static constexpr std::size_t Value = 0; };
template<template <typename...> class C, typename...Ts>
struct NbParams<C<Ts...>> { static constexpr std::size_t Value = sizeof...(Ts); };
constexpr size_t size1 = NbParams<MyTemplateClass<int>>::Value;
constexpr size_t size2 = NbParams<MyTemplateClass2<int, short>>::Value;
That is the regular way one does this kind of things... ;)
Since C++20 concepts aren't standardized yet, I'm using static_assert as a makeshift concept check, to provide helpful error messages if a type requirement isn't met. In this particular case, I have a function which requires that a type is callable before getting its result type:
template <typename F, typename... Args>
void example() {
static_assert(std::is_invocable_v<F, Args...>, "Function must be callable");
using R = std::invoke_result_t<F, Args...>;
// ...
}
In addition, I require that the callable's result must be some kind of std::optional, but I don't know what type the optional will hold, so I need to get that type from it:
using R = // ...
using T = typename R::value_type; // std::optional defines a value_type
However, this will fail if type R doesn't have a value_type, e.g. if it's not a std::optional as expected. I'd like to have a static_assert to check for that first, with another nice error message if the assertion fails.
I could check for an exact type with something like std::is_same_v, but in this case I don't know the exact type. I want to check that R is some instance of std::optional, without specifying which instance it must be.
One way to do that is with a helper trait:
template <typename T>
struct is_optional { static constexpr bool value = false; };
template <typename T>
struct is_optional<std::optional<T>> { static constexpr bool value = true; };
template <typename T>
constexpr bool is_optional_v = is_optional<T>::value;
…and then I can write:
static_assert(is_optional_v<R>, "Function's result must be an optional");
That works, but it seems a little awkward to pollute my namespace with a helper trait just for a one-off check like this. I don't expect to need is_optional anywhere else, though I can imagine possibly ending up with other one-off traits like is_variant or is_pair too.
So I'm wondering: is there a more concise way to do this? Can I do the pattern matching on instances of std::optional without having to define the is_optional trait and its partial specialization?
Following the suggestion by several respondents, I made a re-usable trait:
template <typename T, template <typename...> typename Tpl>
struct is_template_instance : std::false_type { };
template <template <typename...> typename Tpl, typename... Args>
struct is_template_instance<Tpl<Args...>, Tpl> : std::true_type { };
template <typename T, template <typename...> typename Tpl>
constexpr bool is_template_instance_v = is_template_instance<T, Tpl>::value;
…so that I can write:
static_assert(is_template_instance_v<R, std::optional>, "Function's result must be an optional");
This is just as many lines and declarations as the is_optional trait, but it's no longer a one-off; I can use the same trait for checking other kinds of templates (like variants and pairs). So now it feels like a useful addition to my project instead of a kluge.
Can I do the pattern matching on instances of std::optional without having to define the is_optional trait and its partial specialization?
Maybe using implicit deduction guides for std::optional?
I mean... something as
using S = decltype(std::optional{std::declval<R>()});
static_assert( std::is_same_v<R, S>, "R isn't a std::optional" );
Explanation.
When R is std::optional<T> for some T type, std::optional{r} (for an r value of type R) should call the copy constructor and the resulting value should be of the same type R.
Otherwise, the type should be different (std::optional<R>).
The following is a full compiling example.
#include <iostream>
#include <optional>
template <typename T>
bool isOptional ()
{
using U = decltype(std::optional{std::declval<T>()});
return std::is_same_v<T, U>;
}
int main ()
{
std::cout << isOptional<int>() << std::endl; // print 0
std::cout << isOptional<std::optional<int>>() << std::endl; // print 1
}
Anyway, I support the suggestion by super: create a more generic type-traits that receive std::option as template-template argument.
I am trying to learn about SFINAE (i am following this tutorial), but there are some... "design choices" I do not understand and, as such, I find them confusing.
Let's assume I have a situation like this (included re-implementation of std::enable_if is there just to demonstrate how I understand enable_if)
// A default class (class type) declaration. Nothing unusual.
template <bool, typename T = void>
struct enable_if
{};
// A specialisation for <true, T> case. I understand 'why-s' of this.
// -- 'why-s': if I attempt to access 'enable_if<false, T>::type' (which does not exist) I will get a substitution failure and compiler will just "move-on" trying to match "other cases".
template <typename T>
struct enable_if<true, T> {
typedef T type;
};
// Here lies my problem:
template <class T,
typename std::enable_if<std::is_integral<T>::value,T>::type* = nullptr>
void do_stuff(T& t) { /* do stuff */ };
(1) The very 1st thing I have a "problem" with, is bool literal (true/false). I understand they are correct and templates can accept compile-time constant values of primitive data types (plain-old-data types) but if I were tasked to design the enable_if "mechanisms" instead of using true/false I would create a tag classes true_t(or True) and false_t (or False) as follows:
class true_t {}; // or True
class false_t {}; // or False
template<typename T>
class is_integral // just to have "something" to use with "enable_if"
{
using result = false_t;
};
template<>
class is_integral<int32_t> // same with all other int types
{
using result = true_t;
};
template <typename B, typename T = void>
struct enable_if
{};
template <typename T>
struct enable_if<true_t, T>
{
using type = T;
};
(2) The second thing I find redundant is the need to specify typename T template parameter. Wouldn't it be easier / better to just implement enable_if as follows:
template <typename B>
struct enable_if
{};
template <>
struct enable_if<true_t>
{
using type = void; // the 'type' exists therefore substitution failure will not occur.
};
I am well aware that all my propositions are extremely inferior to the currently existing solutions, but I don't understand why... What portion of the functionality (important functionality) of current SFINAE did i shave off? (Not even realizing it...)
I know that, on this site, I am obligated to ask a single question within a... single "question-post-like" format, but if you find it acceptable could I also ask what will this syntax:
std::enable_if</* ... */>::type* = nullptr
accomplish? It's beyond my understanding right now...
The very 1st thing I have a "problem" with, is bool literal (true/false). I understand they are correct and templates can accept compile-time constant values of primitive data types (plain-old-data types) but if I were tasked to design the enable_if "mechanisms" instead of using true/false I would create a tag classes true_t(or True) and false_t (or False) as follows
The issue with use a tag type instead of just a bool is that you have to add extra complexity to the code. If you want to check a compile time condition, like sizeof for instance, you couldn't just do sizeof(T) == 8. You would have to make an abstraction that does the check and the returns the appropriate tag type.
The second thing I find redundant is the need to specify typename T template parameter. Wouldn't it be easier / better to just implement enable_if as follows
Not really. What if you want to use the SFINAE for the return type? You would only be able to have a void function then, which is unnecessarily limiting. Instead what you can do is use what was later added in C++14 and C++17 and make aliases. This makes the names non dependent and lets you drop the typename
template< bool B, class T = void >
using enable_if_t = typename enable_if<B,T>::type;
template< class T >
inline constexpr bool is_integral_v = is_integral<T>::value;
This allows you to rewrite
template <class T,
typename std::enable_if<std::is_integral<T>::value,T>::type* = nullptr>
void do_stuff(T& t) { /* do stuff */ };
to
template <class T,
std::enable_if_t<std::is_integral_v<T>,T>* = nullptr>
void do_stuff(T& t) { /* do stuff */ };
although I prefer to use a bool for the type of enable_if_t like
template <class T,
std::enable_if_t<std::is_integral_v<T>, bool> = true>
void do_stuff(T& t) { /* do stuff */ };
I know that, on this site, I am obligated to ask a single question within a... single "question-post-like" format, but if you find it acceptable could I also ask what will this syntax:
std::enable_if</* ... */>::type* = nullptr
accomplish?
It makes a pointer to the type that std::enable_if "returns" and sets it to null pointer. The goal here is to make a template parameter that will only exist if the condition is true. You could rewrite it to
typename = typename std::enable_if</* ... */>::type
so instead of having a non type parameter you have a type parameter. They both accomplish the same thing but the latter wont work with overloading the function for different enable_if's since default template parameters are not part of the signature. The first version which uses non type parameters is included in the function signature and does allow you to overload the enable_if's.
First off there exists tag-types for true and false, namely std::true_type and std::false_type.
Let's say we made the enable_if work with this instead of a bool parameter. You could then no longer do things like std::enable_if<1 == 1>::type since 1 == 1 evaluates to a bool. So does most things you will want to test here.
On the other hand, the existing tag types can be used in a enable_if since they contain a value and have a operator() that return said value.
So it seems to me that a lot of convenience would be lost in doing it your way, and from what I can see nothing would be gained.
For point 2, it's simply a convenience to be able to specify what type you want enable_if to hold if it's true. It defaults to void, but if you want you can easily have it deduce an int, double ect. which can be useful sometimes.
I'm fiddling around with template metaprogramming, specifically with type sequences and STL-like algorithms working on those sequences. One thing I ran into was the transformation of predicates, for example by binding one of their parameters
I think it's hard to describe my problem without first providing some background information. Here's an example:
#include <type_traits>
// Type sequence
template<class... T>
struct typeseq
{
static constexpr size_t Size = sizeof...(T);
};
// Some algorithm
template<class TS, template<class...> class P>
using remove_if = /* ... some template logic ... */;
// Usage example:
using a = typeseq<int, float, char*, double*>;
using b = remove_if<a, std::is_pointer>; // b is typeseq<int, float>
As shown here, remove_if requires a predicate that serves as an oracle for the removal algorithm to determine which of the elements to remove. As we're dealing with metaprogramming, it is in the form of a template template parameter. ( Note that P is using a variadic template parameter here. Although I'm expecting a unary template, an earlier version of C++ had the restriction that a variadic template argument can't be used as a non-variadic template parameter, which severely restricts predicate transformations. ) It requires that the predicate, when instantiated, resolves to a type that has a nested compile time value member that is convertible to bool.
All is well, but say you want to remove every type that is convertible to int. Obviously, std::is_convertible is a binary predicate, but remove_if above requires a unary predicate. We just need to fix the second template argument to int. Let's define a bind2nd:
template<template<class, class...> class P, class BIND>
struct bind2nd
{
template<class T1, class... Tn> using type = P<T1, BIND, Tn...>;
};
// and now we should be able to do:
using c = remove_if<ts, bind2nd<std::is_convertible, int>::type>; // c is typeseq<char*, double*>;
Unfortunately, this fails to compile on latest Clang and MSVC++. Apparently, the issue is the pack expansion of Tn... into std::is_convertible<T1, BIND, Tn...> while std::is_convertible only has 2 template parameters. It doesn't seem to matter that the pack is empty in practice (isolated example)
I'd rather not provide 'overloads' for any required arity of the predicate passed into bind2nd. Is there a way to detect the arity of P in bind2nd above? GCC allows me to partially specialize it for non-variadic versions of P:
template<template<class, class> class P, class BIND>
struct bind2nd<P, BIND>
{
template<class T1> using type = P<T1, BIND>;
};
But unfortunately GCC wasn't the one that was complaining in the first place. I also doubt how conformant such a partial specialization is. Is there a way around this? Is it possible to detect the actual arity of a template template parameter, and do something different based on that information?
I think I found a workaround.
The problem seems related to type aliases - they seem to directly pass along any template arguments, rather than instantiating the type as is the case with a class or struct. We can use this in our favor, by using a struct as an intermediate step:
template<template<class, class...> class P, class BIND>
struct bind2nd
{
template<class... Tn>
struct impl
{
using type = P<Tn...>;
};
template<class T1, class... Tn> using type = typename impl<T1, BIND, Tn...>::type;
};
Now it works :). It's a bit convoluted though. I wonder if this is all according to standard, but it seems to compile on all major compilers.
Edit: Why am I complicating things by using nested types and aliases? I can just as well use derivation for predicates:
template<template<class, class...> class P, class BIND>
struct bind2nd
{
template<class T1, class... Tn>
struct type : P<T1, BIND, Tn...> { };
};
Clean and simple. And it makes it almost identical to the first definition of bind2nd in the OP.
Maybe it's flu, or I'm just plain stupid, but I can't understand a part of this Crow framework code. My inner C++ parser fails.
template <typename MW>
struct check_before_handle_arity_3_const
{
template <typename T,
//this line
void (T::*)(int, typename MW::context&) const = &T::before_handle
>
struct get
{ };
};
I know it's a template parameter inside template declaration. It looks like maybe some lambda or function pointer type parameter... but, I'm not sure.
Can someone explain this line?
Update:
Exploring depths of newly obtained knowledge - after the answer was given - led me to this excerpt from a great book:
A template can accept a pointer to a function as a nontype template
parameter. (Most often in this book, nontype template parameters are
integral values.) [...] Using a pointer to a function as a nontype
template argument means that we no longer need to store it in the map.
This essential aspect needs thorough understanding. The reason we
don't need to store the pointer to a function is that the compiler has
static knowledge about it. Thus, the compiler can hardcode the
function address in the trampoline code.
So, now I know one of the reasons to use this technique.
void (T::*)(int, typename MW::context&) const is a non-type template parameter.
It is a pointer to a member function of T.
With the use of = &T::before_handle, its default value is set to &T::before_handle.
The reason I used this technique is to support 2 kinds of handler format:
https://github.com/ipkn/crow/blob/master/include/middleware.h#L17
check_before_handle_arity_3_const is used here:
template <typename T>
struct is_before_handle_arity_3_impl
{
template <typename C>
static std::true_type f(typename check_before_handle_arity_3_const<T>::template get<C>*);
template <typename C>
static std::true_type f(typename check_before_handle_arity_3<T>::template get<C>*);
template <typename C>
static std::false_type f(...);
public:
static const bool value = decltype(f<T>(nullptr))::value;
};
With SFINAE, is_before_handle_arity_3_impl<T>::value is determined whether we can call the handler with 3 arguments or not. By using std::enable_if, Crow can call the proper handler:
https://github.com/ipkn/crow/blob/master/include/http_connection.h#L110