Select a variable at compile-time

Question

I'd like to be able to specify via a boolean which of two variables I need to use at compile time, all of this without direct SFINAE . Just one single function, similar to std::conditional but not returning types.

So for instance in a class test , I'd like to have

class test
{
    template <class T, bool first, MAGIC_HAPPENS var>
    void apply_it(T &t)
    {
        for (auto &p : var) p.apply(t);
    }

    std::vector<myfunction> var1;
    std::vector<myfunction> var2;
}

The use is simple: if I specify first == true then it should apply the loop with var == var1 , otherwise var == var2 .

Is it possible?

Answer 1

Just for fun(*), a minimal C++17(**) modification to your current code snippet might be

class test
{
    std::vector<myfunction> var1;
    std::vector<myfunction> var2;

    template <class T, bool first, std::vector<myfunction> test::*var = first ? &test::var1 : &test::var2 >
    void apply_it(T &t)
    {
        for (auto &p : this->*var) p.apply(t);
    }
};

---

(*) I see no situation in which such a thing would be preferable over the other suggested solutions ...

(**) as far as I know, this requires C++17 due to linkage requirement of template non type pointer parameters ...

Answer 2

For C++17 and above:

class test
{
    template <class T, bool first>
    void apply_it(T &t)
    {
        if constexpr (first)
        {
            for (auto &p : var1) p.apply(t);
        }
        else
        {
            for (auto &p : var2) p.apply(t);
        }
    }

    std::vector<myfunction> var1;
    std::vector<myfunction> var2;
}

if constexpr is evaluated at compile time if the condition is constexpr, which is the case for template parameters.

Answer 3

You can do that in C++11 with a pointer to data member:

class test
{
    template <class T, bool first>
    void apply_it(T &t)
    {
        constexpr auto var = first ? &test::var1 : &test::var2;
        for (auto &p : this->*var) p.apply(t);
    }

    std::vector<myfunction> var1;
    std::vector<myfunction> var2;
}

Answer 4

template<std::size_t I, class...Args>
decltype(auto) pick( Args&&... args ) {
  return std::get<I>( std::forward_as_tuple( std::forward<Args>(args)... ) );
}

pick selects at compile time something from a list.

class test
{
  template <bool first, class T>
  void apply_it(T&& t)
  {
    auto&& var = pick<first?0:1>(var1, var2);
    for (auto&& p : var)
      p.apply(t);
  }

  std::vector<myfunction> var1;
  std::vector<myfunction> var2;
};

I also made some minor improvements while I was copy pasting.