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.