Enum variable as a template parameter

I have a enum class:

enum class flipping_t {
    eNone,
    eHorizontal,
    eVertical
};

Then I’m trying to use its values as a template parameters:

template<flipping_t>
void areas();


template<>
void areas<flipping_t::eHorizontal>();


template<>
void areas<flipping_t::eVertical>();

My app has a variable of type flipping_t:

auto flip = flipping_t::eNone;

I want to change its value depending on which key has just been pressed down (I’m working on simple photo editor so want to mirror a picture)

Then I’m invoking a function according to current flip:

areas<flip>();

And compiler says:

no instance of function template "areas" matches the argument

I understand why areas<flipping_t::eNone>() works fine. I’m looking for a way to fix areas<flip>()

>Solution :

A template parameter can be of an integral type, what flipping_t is, but is shall be a constant. That is the reason why areas<flipping_t::eNone>() works fine: areas<flipping_t::eNone>() is indeed a (compile time) constant.

But when you declare auto flip = flipping_t::eNone;, flip is a variable, and its values is allowed to change at run time. That is the reason why you cannot use it as a template parameter.

If it can be constant, you could say it to the compiler:

    const auto flip = flipping_t::eHorizontal;

This is enough to later be able to use areas<flip>(). But in that case, flip is nothing more than an alias to flipping_t::eHorizontal