How can I use a enum variable as template argument?

Question

If we have an enumeration type:

enum E {
   E1,
   E2,
   // ...
};

and based on E, a class template:

template <E T>
class C { /* ... */ };

Is there a way of using a declared variable of the type E as a template argument?

Example code:

E example_type = E1;
C<example_type> example_class;

Perhaps this answers your question? It uses a function parameter instead of a local/global variable. — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Dec 26, 2020 at 16:56
Could you clarify your example code? Currently it's not clear what you want, and your use of C makes no sense — alter_igel
– alter_igel, Commented Dec 26, 2020 at 16:58
Depending on a string, I need to return an enum type. I want to use that type directly in another method. — Stukata
– Stukata, Commented Dec 26, 2020 at 23:18

alex_noname · Accepted Answer · 2020-12-26 17:14:19Z

3

For integral (which an enumeration is) and arithmetic types, the template argument provided during instantiation must be a constant expression. For example:

enum E {
   E1,
   E2,
};


template <E enum_val>
class Foo {
};

int main() {

    constexpr E var = E1;
    const E var2 = E2;
    Foo<var> foo;
    Foo<var2> foo2;

    E var3 = E2;
    Foo<var3> foo3;  // error: the value of ‘var3’ is not usable in a constant expression 
}

answered Dec 26, 2020 at 17:14

alex_noname

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