how to use a rvalue reference in a switch's case

Question

I want to use t(rvalue reference) in a case of switch(T), but I get an error the value of 't' is not usable in a constant expression. How to do it the right way.

#include <iostream>
using namespace std;
int main(){
    int (&&t)=5;
    int T{};
    switch(T){
        case t:   // error in this case
            cout<<t<<endl;
            break;
        default:
            cout<<"default"<<endl;
            break;
    }
}

Answer 1

Note that case labels in a switch statement need to be constant expressions.

You can't use an rvalue reference as a case label, since it is not a constant expression. You can however use a compile time initialized variable:

constexpr int t = 5;
switch(...)
{
  case t : ...  // ok
}

Note that trying to do something like:

int const &&t = 5;

will not work either, since the initializer is not a constant expression, so there is no way that I'm aware of that lets you use an rvalue reference as a case label.

Note also, that something like:

int const t = ...

will not work unless the initializer is a constant expression (i.e. known at compile time). So:

int const t = 5;

will work, but:

int n = 5;
int const t = n;

will not.