How to check that an element is in a std::set?

The typical way to check for existence in many STL containers such as std::map, std::set, ... is:

const bool is_in = container.find(element) != container.end();

Another way of simply telling if an element exists is to check the count()

if (myset.count(x)) {
   // x is in the set, count is 1
} else {
   // count zero, i.e. x not in the set
}

Most of the times, however, I find myself needing access to the element wherever I check for its existence.

So I'd have to find the iterator anyway. Then, of course, it's better to simply compare it to end too.

set< X >::iterator it = myset.find(x);
if (it != myset.end()) {
   // do something with *it
}

C++ 20

In C++20 set gets a contains function, so the following becomes possible as mentioned at: https://stackoverflow.com/a/54197839/895245

if (myset.contains(x)) {
  // x is in the set
} else {
  // no x 
}

Just to clarify, the reason why there is no member like contains() in these container types is because it would open you up to writing inefficient code. Such a method would probably just do a this->find(key) != this->end() internally, but consider what you do when the key is indeed present; in most cases you'll then want to get the element and do something with it. This means you'd have to do a second find(), which is inefficient. It's better to use find directly, so you can cache your result, like so:

auto it = myContainer.find(key);
if (it != myContainer.end())
{
    // Do something with it, no more lookup needed.
}
else
{
    // Key was not present.
}

Of course, if you don't care about efficiency, you can always roll your own, but in that case you probably shouldn't be using C++... ;)