Can this C++ template be simplified with a C MACRO?

Question

I have a template class with a read() method. It needs to call a C functions: read1(), read2(), read3() etc based upon the template parameter value. Is there is a cleaner way to do this than a switch on the template value? I have tried using a MACRO:

#define READ(N) read##N##()

But this doesn't work as X is simply replaced by the C Preprocessor as 'N' resulting in "readN()"

int read1()
{
}

int read2()
{
}

int read3()
{
}

...

template <int N>
struct Reader
{
    int read()
    {
        switch (N)
        {
        case 1:
            return read1();
        case 2:
            return read2();
        case 3:
            return read3();
        }
    }

    /* Doesn't work as X is simply replaced by the C Preprocessor as 'N' resulting in "readN()"
    #define READ(X) read##X##()
    int read()
    {
        return READ(N);
    }
    */
};

Answer 1

One can

1. use if constexpr instead of switch to select proper read call:

2. use constexpr lookup table, for neatness.

3. use, if desperate, constexpr implementation of map, but standard ISO one is too generic and cannot be constexpr. A map is more useful for run-time dispatching.

4. wrap functions into specializations of a template function, if there are only few.

Lookup table as a function pointer table:

#include <iostream>

int read1(){ return 1; }
int read2(){ return 2; }
int read3(){ return 3; }

template <int N>
struct Reader
{
    static constexpr int (*read_functions[])(void) = {
      nullptr, 
      read1,
      read2,
      read3,
    };

    static_assert(N > 0 && N < std::size(read_functions));
    int read() {
        return read_functions[N]();
    }
};

int main()
{
    Reader<3> r;
    std::cout << "Read " << r.read();
}

See on godbolt. Note, with -O2 it's completely optimized out.