Check whether an element is in std::initializer_list

If you have access to c++20 you can use set's contains which returns a bool allowing you to do:

if(set{ 4, 8, 15, 16, 23, 42 }.contains(x))

Live Example

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Otherwise, with just c++11 you can still use set's count which only returns 1 or 0 allowing you to do something like:

if(set<int>{ 4, 8, 15, 16, 23, 42 }.count(x) > 0U)

Live Example

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Keep in mind that magic numbers can be confusing for your audience (and cause 5 seasons of Lost.)
I'd recommend declaring your numbers as a const initializer_list<int> and giving them a meaningful name:

const auto finalCandidates{ 4, 8, 15, 16, 23, 42 };

if(cend(finalCandidates) != find(cbegin(finalCandidates), cend(finalCandidates), x))

boost::algorithm::contains doesn't only work on strings, it works on any range, i.e. a sequence that can yield a begin and end iterator. To find a single value use it as follows:

auto l = {1,2,3,4};
auto l1 = {2};      // thing you want to find
if(boost::algorithm::contains(l, l1)) { ... }

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You can perform your search using the standard library only, but doing so is quite a bit more verbose. A couple of options are:

1. using a lambda

   if(std::any_of(l.begin(), l.end(), 
                  [](int i){ return i == 2; })) { ... }
   

2. using std::bind

   using std::placeholders::_1;
   if(std::any_of(l.begin(), l.end(), 
                  std::bind(std::equal_to<>(), 2, _1)) { ... }
   

Live demo

Note that std::equal_to<>() is a C++14-only option. For a C++11 compiler, use std::equal_to<int>().