CODEWITHSUNDEEP
c++templates
iammilind
iammilind

Reputation: 69988

Knowing the number of template parameters in the declaration

If I have,

template<typename T1, typename T2, int N>
class X {};

Is there any way, that I can know class X has 3 template arguments ?

Use case in brief: There are two library classes ptr<T> (for normal pointer) and ptr_arr<T,N> (for pointer to array). These two are interacting with another class in following way:

template<typename T>
void Clear(const T &obj)
{
  if(T::Args == 1) destroy(obj);
  else destroy_arr(obj);
}

So, I thought if we have some handy way of knowing the number of parameters, it would make it easy. However, I learn that I need to change my business logic as there cannot be such way.

Upvotes: 3

Views: 154

Answers (3)

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506877

There is no way to do this. Imagine the amount of overloads. 

template<int> struct A;
template<bool> struct B;
template<char> struct C;
template<typename> struct D;
template<D<int>*> struct E;
template<D<bool>*> struct F;
template<typename, int> struct G;
// ...

For each of that, you would need a different template to accept them. You cannot even use C++0x's variadic templates, because template parameter packs only work on one parameter form and type (for example, int... only works for a parameter pack full of integers).

Upvotes: 0

spraff
spraff

Reputation: 33395

There is no standard way to do this (unless you use variadic sizeof(Args...) in C++0x) but that's beside the point -- the question is wrong.

Use overload resolution.

template <typename T>
void clear (ptr<T> & obj) {
    destroy (obj);
}

template <typename T, int N>
void clear (ptr_arr<T,N> & obj) {
    destroy_arr (obj);
}

Upvotes: 6

Anycorn
Anycorn

Reputation: 51445

You can use the mpl::template_arity (undocumented)

http://www.boost.org/doc/libs/1_40_0/boost/mpl/aux_/template_arity.hpp

Upvotes: 5

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