CODEWITHSUNDEEP
c++templatesc++14template-specialization
L117
L117

Reputation: 586

Virtual method with unspecialized template argument

#include <iostream>
#include <array>
#include <vector>

using namespace std;

// Currently I have code much like this one:

template <const uint32_t N>
using VectorN = array<double, N>;


template <const uint32_t N>
class ITransformable {
public:
    virtual vector<VectorN<N>>&  positions() = 0;
};


class SomeTransformer {
public:
    template <const uint32_t N>
    void operator()(ITransformable<N>& transformable) const {
        /* implementation */
    }
};

// Then I want to create interface like this.

template <const uint32_t N>
class ITransformer {
public:
    virtual void operator()(ITransformable<N>& transformable) const = 0;
};

// And finally implement it for SomeTransformer:
// 
// Notice that class is not template, this is intentional.
//
// class SomeTransformer : public ITransformer<N> {
// public:
//     virtual void operator()(ITransformable<N>& transformable) const {
//         /* implementation */
//     }    
// }

Actually, now it seems impossible to me. Otherwise this class would inherit indefinite number of interface specializations...

But still, is it possible, at least for finite number of dimensions N?

template <template <typename> class C> seems to be related, but I can't figure out how to apply this.

EDIT What I want is something like this:

class SomeTransformer : 
    public ITransformer<2>, 
    public ITransformer<3>, 
    public ITransformer<4>, 
    ..., 
    public ITransformer<N> { 
    /* ... */ 
};

For any N ever used in code. This seems impossible, as I said.

Upvotes: 1

Views: 73

Answers (2)

bartop
bartop

Reputation: 10315

You can achieve what you want or nearly that. Here's what I propose:


#include <type_traits>
#include <utility>

template<std::size_t N>
struct ITransformer {};

template<class T>
class SomeTransformer_h { };

template<std::size_t... Indices>
class SomeTransformer_h<
    std::integer_sequence<std::size_t, Indices...>> :  
    public ITransformer<1 + Indices>... { };


template<std::size_t N>
class SomeTransformer : public SomeTransformer_h<
    std::make_index_sequence<N>
> { };

int main() {
    SomeTransformer<5> a;
    ITransformer<1>& ref = a;
    ITransformer<4>& ref2 = a;
    ITransformer<5>& ref3 = a;
}

Now for any N it will make SomeTransformer inherit all ITransformer from 1 to N.

Upvotes: 1

NutCracker
NutCracker

Reputation: 12263

Since N is not declared anywhere, you cannot use it. You need something like:

class SomeTransformer : public ITransformer<5> {
public:
    virtual void operator()(ITransformable<5>& transformable) const {
        /* implementation */
    }    
};

or make it a template class:

template <uint32_t N>
class SomeTransformer : public ITransformer<N> {
public:
    virtual void operator()(ITransformable<N>& transformable) const {
        /* implementation */
    }    
};

UPDATE

There is no dynamic inheritance in C++. Therefore, what you want to achieve is not possible.

Upvotes: 1

Related Questions