How to compare string when precompile

Question

#include <iostream>

#define MY_MACRO "hello"

int main() {
    #if MY_MACRO == "asdasd"
        std::cout << "The macro is not equal to hello" << std::endl;
    #else
        std::cout << "The macro is equal to hello" << std::endl;
    #endif
    return 0;
}

In this form it would not compile successfully.

#include <iostream>

#define MY_MACRO hello

int main() {
    #if MY_MACRO == asdasd
        std::cout << "The macro is not equal to hello" << std::endl;
    #else
        std::cout << "The macro is equal to hello" << std::endl;
    #endif
    return 0;
}

And in this form #if MY_MACRO == XXX would always judge as true.

So how to compare strings when pre-complie?

Answer 1

You have two options:

1. Use numbers instead of strings, and give them names using macros:

   #define HELLO 1
   #define ASDASD 2
   
   #define MY_MACRO HELLO
   
   #if MY_MACRO == HELLO
   // ...
   #endif
   

2. Don't use preprocessor, use a plain if (or if constexpr, the only difference here is that the latter forces the condition to be a compile-time constant). This means that all branches have to be valid (have to compile) regardless of which one is choosen:

   #define MY_MACRO "hello"
   
   int main()
   {
       if constexpr (MY_MACRO == std::string_view("hello"))
       {
   
       }
   }
   

   Or make MY_MACRO a global constexpr std::string_view variable if you don't need to override it with compiler flags.