Variadic macro for checking if variable equals one of variadic arguments

I’m currently working on a WebGL-like OpenGL wrapper in , which involves verifying that arguments are actually valid. The problem, more or less, is that OpenGL has a ton, and I’m not exaggerating here, of valid arguments for certain functions. (Image of a naive conditional (possible internalformat constants for glTexImage2D, if you’re interested. I literally wrote a JS table parser/conditional maker for a few of those) to make my point. That’s an assert…)

The logic of such checks is simple enough:

bool check(const unsigned int var)
{
    return var == CONSTANT_1 || var == CONSTANT_2 || var == CONSTANT_...;
}

But, as per the above example, this can stretch for up to 84 possible constants, which is… yeah. And I’m fully aware that a macro wouldn’t cut this down by an absurd amount, but it would still make a difference, and I feel like it would be cleaner too. Not to mention the possibility of other macros of a similar fashion with different operators.

So my idea was to use a macro. After all, this check would be trivial to do at runtime, with a std::initializer_list``, std::vector, std::array` or some other container, but since all of these constants are known at compile time, I feel as though this is unnecessary.

Since the number of possible constants is variable, I see no way of not using a variadic macro. Yet I don’t know how I would accomplish this. One possible way I thought of doing this is basically overloading a macro based on the number of arguments (akin to this), but this seems unnecessarily complex.

In essence, I’m looking for a macro that would fulfill the following:

MACRO(var, CONSTANT_1, CONSTANT_2, CONSTANT_3)

expands to

var == CONSTANT_1 || var == CONSTANT_2 || var == CONSTANT_3

with any number of constants (important!, and the hard part).

Answer

You might want to use C++17 fold expression:

template<class... Args>
bool check(const unsigned int var, const Args&... args)
{
  return ((var == args) || ...);
}

Then you can invoke something like this:

check(var, CONSTANT_1, CONSTANT_2, CONSTANT_3);