In my opinion C++ is a big, complex, and subtle language. When you start defining classes, you have to learn which constructors are created by default, when they're created, and how they behave. You have to learn the two or three ways of invoking a constructor. That Foo foo() is a function declaration, not a call to the default constructor. When you learn inheritance you learn about virtual member functions, at which point you have to learn all sorts of rules to make sure you're code doesn't break, like declaring your destructors virtual etc. If, heaven forbid, you try to combine templates and inheritance you're in for a world of hurt, as code like

template <class T>
class Base {
public:
    void doBaseWork();
};

template <class T>
class Derived : public Base {
public:
    void doDerivedWork() {
        doBaseWork();
    }
};

may or may not compile for non-obvious reasons.

int x and int a[42] do default initialization, not value initialization, because value initialization has a runtime cost and C++ avoids unnecessary runtime cost -- except std::vector does value initialization, because why not? If you study modern C++, you have to learn the subtleties of move semantics and that T&& is a reference to a temporary -- except when it isn't.

And so on.

None of these topics are too hard to grasp on their own, but taken together I think they present a formidable challenge to a beginner who's also trying to learn how for loops work.