take template basis on run-time value

Question

I have an enum:

enum operation {
 plus,
 delete
 //...
}

There is function, which has a run-time argument.

operation_do(plus);

And inside that function, there is a template-function call, which is based on a run-time argument. Important: I can NOT make operation_do(), as a template function. This is the condition.

void operation_do(operation op) {
    call<op>();
}

I have a compiled error: op is not a constant expression.

I tried to resolve this with the help function, like this:

constexpr operation get(operation arg) {
    return arg;
}

void operation_do(operation op) {
    constexpr operation elem = get(op);
    call<elem >();
}

But it is still the same error: op is not a constant expression. Can anyone help, please, with this?

Answer 1

And inside that function, there is a template-function call, which is based on a real-time argument. Important: I can NOT make operation_do(), as a template function. This is the condition.

This is the problem.

In C++ a run-time value (so a function argument, that can be a run-time value) can't be used as template parameter.

For the same reason can't works the passage through a constexpr value

void operation_do(operation op) {
    constexpr operation elem = get(op); // <--- error
    call<elem >();
}

because elem can't be initialized from a run-time value.

The best I can imagine is the use of a sequence of if (or, maybe better, a switch) to pass from a run-time value to a compile-time value.

Something as

void operation_do (operation op)
 {
   switch ( op )
    { 
      case plus:
         call<plus>();
         break;

      case del:
         case<del>()
         break;
    }
 }

Obviously this is acceptable only if the number of the enum value is limited and if you can concentrate the switch in a single place.

Note that delete is a keyword, so you can't use it as an identifier.