What's the most C++ way to check if value belongs to certain static set?

Question

Let's say that I want to write something like this (the {1, 3, 7, 42, 69, 550123} set is known before compilation):

int x;
...
if (x == 1 || x == 3 || x == 7 || x == 42 || x == 69 || x == 5550123)
{
  ...
}

The condition looks ugly because we have 9 extra symbols ("|| x ==") for each possible value. How can I rewrite it in a more C++ way?

My best guess is:

int x;
...
const std::unordered_set<int> v = {1, 3, 7, 42, 69, 5550123};
if (v.count(x))
{
  ...
}

It has O(1) average complexity with some memory and time overhead, but still looks kinda ugly.

Answer 1

The only clean way to do this is to simply move it to a method. Name the method appropriately and it really does not matter what you do inside.

bool is_valid_foo_number(int x)
{
    return x == 1
        || x == 3
        || x == 7
        || x == 42
        || x == 69
        || x == 5550123;
}

The method looks good enough for me, because all I will ever see of it is

if (is_valid_foo_number(input))
{
    // ...
}

Should a technical detail change (like the sheer amount of valid numbers requiring another lookup approach or maybe a database instead of hardcoded values) you change the method's internals.

The point is that I think it only looks ugly... because you need to look at it while you look at your logic. You shouldn't have to look at the details anyway.