Hierarchical Inheritance is when multiple derived classes inherit from a single base class. All the derived classes share the features of that one common base class. This kind of inheritance structure resembles a hierarchy or tree structure.

In C++, an abstract class is defined by having at least one pure virtual function. This means the function is declared but not defined within the class, ensuring that derived classes must implement it.

In C++, both structs and classes can have member functions, inherit from others, and serve many of the same roles. The primary difference is the default access level (structs default to public; classes default to private). However, structs are conventionally used for simple data storage without associated behavior or methods.

In C++, structs are typically used for lightweight user-defined data types. They default to public member access, making it easier to access the members directly without the need for getters and setters. Classes default to private access and are generally used when encapsulation is a primary concern.

The "Rule of Three" in C++ suggests that if a class requires a user-defined destructor, copy constructor, or assignment operator, it likely requires all three. This rule arises due to the intricacies of dynamic resource management (like memory allocation) in C++. Implementing or omitting one without the others can lead to unexpected behavior.

The diamond problem arises in C++ multiple inheritance when one class inherits from two classes that have a common base class. This can lead to ambiguity regarding which base class member should be accessed. The "virtual" keyword before the base class prevents the compiler from creating multiple instances of the base class, thus resolving the diamond problem.

The "&" (ampersand) operator is used to get the memory address of a variable in C++. For instance, if int x = 10;, then &x will give the memory address where the integer x is stored. The asterisk (*), on the other hand, is used for dereferencing.

Both struct and class in C++ can have member functions, data members, and can support inheritance. The primary difference is the default access level: members of a struct are public by default, whereas members of a class are private. Other functionalities between them are essentially identical.

While erase can remove elements from a vector, to efficiently remove all instances of a value, you'd first use remove to move those elements to the end and then use erase. This is known as the "erase-remove" idiom in C++.

In C++, the "base" keyword is not actually used. The correct way to access base class members is through the use of the base class name or the scope resolution operator. However, "base" is a term from C#.