C++ enum class as a variable template parameter

Question

I'm trying to optimize out some function at compile time by using an enum class as a template parameter.

Say for example

enum class Color { RED, BLACK };

Now, I would like to define a method

void myMethod<Color c> () {
 if( c == Color::RED ) { ... }
 if( c == Color::BLACK ) { ... }
}

And I would like the compiler to make 2 copies of myMethod and eliminate the dead code during the optimisation (it's for CUDA kernels so speed and register usage is important to me)

However, seems like when I call the method using

void doSomething( const Color c ) {
 myMethod<c>();
}

MSVC complains with "expression must have a constant value". I was expecting the compiler to be clever enough to compile a version of myMethod with each possible version of the enum. Is that not the case ? Can I force it to, without an ugly switch in doSomething ?

Thanks for your help !

Somewhere you have to decide in run-time which myMethod<Color>() you want to call, if it's dependent of a non-constant variable. So, you are back to a switch() (or an if-else-cascade, or an array mapping colors to function pointers, or something else). — Scheff's Cat
– Scheff's Cat, Commented Feb 26, 2020 at 8:54
This seems to happen often recently. const doesn't make a (compile time) constant expresion, i.e. a expression that can be evaluated at compile time. — Lukas-T
– Lukas-T, Commented Feb 26, 2020 at 8:54
in a nutshell: template = compile time, ordinary function parameters = runtime. You can mix, but then you need to define how to mix, eg as already mentioned with a switch to choose which function to call — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Feb 26, 2020 at 8:56
Fair enough, but irrelevant of the const expression, I still thought the compiler would somehow read out my enum and decide at runtime based on the value of the argument. Ok maybe I expect too much... Thank you for the help :) — Norman Pellet
– Norman Pellet, Commented Feb 26, 2020 at 8:59
What you might want to achieve, I once applied to a fun project to move ifs out of the most inner loops to the outside. Of course, the nested loops had to be re-implemented for every case. There I used the template-with-value-parameters to prevent code duplication. the template, the call of that template and a little explanation Optimization Attempts. — Scheff's Cat
– Scheff's Cat, Commented Feb 26, 2020 at 9:06

Adrian Mole · Accepted Answer · 2022-05-18 14:52:26Z

You have to decide between run-time or compile-time evaluation. The compile-time version can be something like this:

enum class Color { RED, BLACK };

template <Color c>
void myMethod () {
    if constexpr ( c == Color::RED ) { std::cout << "RED" << std::endl; }
    if constexpr ( c == Color::BLACK ) { std::cout << "BLACK" << std::endl; }
}   

int main()
{   
    myMethod<Color::RED>();
    myMethod<Color::BLACK>();
}

But, if you need to evaluate the variable at run-time, you have to switch over all of the possible values:

enum class Color { RED, BLACK };

template <Color c>  
void myMethod () {
    if constexpr ( c == Color::RED ) { std::cout << "RED" << std::endl; }
    if constexpr ( c == Color::BLACK ) { std::cout << "BLACK" << std::endl; }
}

void RuntimeDispatch( Color c ) 
{
    if ( c == Color::RED ) { myMethod<Color::RED>(); }
    if ( c == Color::BLACK ) { myMethod<Color::BLACK>(); }
}   

int main()
{   
    RuntimeDispatch( Color::RED );
    RuntimeDispatch( Color::BLACK );
}

There is simply no way to use a run-time variable as a template parameter, because the value is not a known constant at compile-time.

If you have to use an older compiler, you can replace constexpr if with template specialization:

template <Color c> void myMethod ();

template <> void myMethod<Color::RED>() { std::cout << "RED" << std::endl; }
template <> void myMethod<Color::BLACK>() { std::cout << "BLACK" << std::endl; }

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