How do I use constexpr logic?

Question

I would like to implement some constexpr logic in some piece of code. I am able to compile and execute this code without problem.

#include <iostream>

int main(){
    std::cout << "Starting c++ main" << std::endl;

    constexpr int val_1 = 100;
    int val_2;

    if constexpr (val_1 == 100){
        val_2 = 10;
    }
    else if constexpr (val_1 == 200){
        val_2 = 20;
    }

    std::cout << val_2 << std::endl;
}

But really, the val_2 should be a constexpr too. How can I get val_2 to be a constexpr? I tried several things, but with no success. If I declare val_2 as constexpr, then I am not allowed to change its value in the if statement. If I do not declare the constexpr variable before the body of the if statements, then it is (as expected) not defined outside of the if statement.

Answer 1

You can put the initializer in a ternary expression:

constexpr int val_2 = val_1 == 100 ? 10 : val_1 == 200 ? 20 : 0;

---

If your initialization logic is more complex you can write a constexpr function like this:

constexpr auto f(int val_1) 
{
  int val_2{};

  if (val_1 == 100){
    val_2 = 10;
  }

  else if (val_1 == 200){
    val_2 = 20;
  }

  return val_2;
}

and then:

constexpr int val_2 = f(val_1);

---

Also, note that if constexpr should only be used to conditionally compile code. You don't need it for regular control flow.

Answer 2

You can use an immediately called lambda:

constexpr int val_2 = []() {
    if constexpr (val_1 == 100){
        return 10;
    }
    else if constexpr (val_1 == 200){
        return 20;
    }
}();