C++: Reference/pointer to member variable as template parameter

Question

To start, I have something like this:

class Test {
    std::vector<int> a, b;
    void caller(...) { callee(...); }
    void callee(...) { /* Do stuff with 'a' */ }
}

What I wanted is to have a function that does exactly the same as callee but for vector b. To do this there are two obvious solutions:

• Pass vector a or b as argument. However, callee is a recursive function that can go for hundreds of calls, and passing the vectors as arguments would just be unnecessary overhead.
• Copy the function callee and use vector b, which would be the best alternative, despite the fact that callee is quite a long function and I would have a lot of duplicate code.

Out of curiosity, I went looking for the templates part and I noticed that can be used for

lvalue reference type

pointer type

pointer to member type

So I tried to do this:

class Test {
    std::vector<int> a, b;
    void caller(...) { callee<a>(...); }
    template <std::vector<int> &x> void callee(...) { /* Do stuff with 'x' */ }
}

but I get

error: use of ‘this’ in a constant expression

Is there any way to achieve this either with a reference or a pointer?

By the way, what I want can be seen as a function-scoped #define

Answer 1

Arrays and even tuples, but no love for good old pointers-to-members ?

class Test {
    std::vector<int> a, b;

    void caller(/*...*/) { callee<&Test::a>(/*...*/); }

    template <std::vector<int> Test::*vec>
    void callee(/*...*/) { /* Do stuff with `(this->*vec)` */ }
};

Answer 2

You cannot use a reference to a data member as a template argument: templates are compile-time, and the value of this is not known until runtime. In other words, you'd need a separate instantiation (separate binary code) for each runtime object of type Test.

What you can do is replace a and b with an array, and templatise callee by index into this array:

class Test {
    std::array<std::vector<int>, 2> ab;
    void caller(...) { callee<0>(...); }
    template <size_t idx>
    void callee(...) { /* Do stuff with 'ab[idx]' */ }
}

This way, you get only two instantiations of callee (one for 0 and one for 1), with the indexing done (or at least doable) at compile time.