Question

Can you write preprocessor directives to return you a std::string or char*?

For example: In case of integers:

#define square(x) (x*x)

int main()
{
   int x = square(5);
}

I’m looking to do the same but with strings like a switch-case pattern. if pass 1 it should return “One” and 2 for “Two” so on..

Answer 1

You don’t want to do this with macros in C++; a function is fine:

char const* num_name(int n, char const* default_=0) {
  // you could change the default_ to something else if desired

  static char const* names[] = {"Zero", "One", "Two", "..."};
  if (0 <= n && n < (sizeof names / sizeof *names)) {
    return names[n];
  }
  return default_;
}

int main() {
  cout << num_name(42, "Many") << '\n';
  char const* name = num_name(35);
  if (!name) { // using the null pointer default_ value as I have above
    // name not defined, handle however you like
  }
  return 0;
}

Similarly, that square should be a function:

inline int square(int n) {
  return n * n;
}

(Though in practice square isn’t very useful, you’d just multiply directly.)

---

As a curiosity, though I wouldn’t recommend it in this case (the above function is fine), a template meta-programming equivalent would be:

template<unsigned N> // could also be int if desired
struct NumName {
  static char const* name(char const* default_=0) { return default_; }
};
#define G(NUM,NAME) \
template<> struct NumName<NUM> { \
  static char const* name(char const* default_=0) { return NAME; } \
};
G(0,"Zero")
G(1,"One")
G(2,"Two")
G(3,"Three")
// ...
#undef G

Note that the primary way the TMP example fails is you have to use compile-time constants instead of any int.