Get number of template parameters with template template function

Question

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

Answer 1

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:

1. 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...

2. 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).

3. 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...

4. 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...

5. 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.

Answer 2

#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.

Answer 3

° 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... ;)