Variadic templates: One method per template argument

Question

Is there a way to automatically create one method per variadic template argument?

For example, in the code below, I want to be forced to override void x(a &v) and void x(b &v) in class i:

#include <type_traits>
#include <stdlib.h>
#include <stdio.h>

class a {
};

class b {
};

template <typename ...T>
class t {
public:
    virtual void x(T &v) = 0;
};

class i : public t<a, b>
{
};

int
main (int argc, char *argv[])
{
    i ii;
    return 0;
}

NathanOliver · Accepted Answer · 2020-10-28T20:45:13.280

4

You can make a t_impl that holds the virtual function for a single T like

template <typename T>
class t_impl
{
public:
    virtual void x(T &v) = 0;
};

and then t would inherit from it like

template <typename ...T>
class t : t_impl<T>... // can use public, protected or private inheritance
{
public:
    using t_impl<T>::x...; // used to import t_impl<T>::x into the public space
    // any additional common members
};

edited Oct 28 '20 at 20:45

answered Oct 28 '20 at 20:32

NathanOliver

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but to make `x()` usable given `t*`, you should add a `using` declaration. – Eugene Oct 28 '20 at 20:39
Nice. This is more efficient than what I suggested (for compilation). We should also note that either the user can't list the same type twice, or we'd need to add something here to prevent a second inheritance in that case... – Elliott Oct 28 '20 at 20:42
@Eugene Good call. Code fixed. – NathanOliver Oct 28 '20 at 20:45
@Elliott Yeah. Adding a static assert with a check from [here](https://stackoverflow.com/questions/18986560/check-variadic-templates-parameters-for-uniqueness) should fix that. – NathanOliver Oct 28 '20 at 20:47

score 1 · Answer 2 · answered Oct 28 '20 at 20:31

Instead of making t a variadic template, just template it on a single type:

template <typename T>
class t {
public:
    virtual void x(T &v) = 0;
};

and inherit base classes like this instead:

class i : public t<a>, t<b>
{
    virtual void x(a &v) {}
    virtual void x(b &v) {}
};

Here's a demo.

Variadic templates: One method per template argument

2 Answers2