Recursion is particularly suitable for problems that exhibit repetitive or nested structures. Examples include problems like calculating factorial, traversing trees, or solving the Towers of Hanoi. Such problems can be broken down into smaller instances of the same problem, making them perfect candidates for a recursive approach.

When a function has a default argument, it can be called with fewer arguments than it defines. The compiler will use the default value for any arguments not provided by the caller. This feature aids in providing flexibility while calling functions but doesn't directly enhance runtime performance.

Hierarchical inheritance is where a single class serves as a base class (superclass) for more than one derived class (subclass). In this scenario, the common attributes and methods of users can be part of the base class, while unique functionalities for Admin, Buyer, and Seller can be defined in their respective derived classes.

A copy constructor in C++ is called in several scenarios: 1) when an object is passed (by value) to a function, 2) when an object is returned (by value) from a function, 3) when an object is constructed based on another object of the same class, and 4) during the initialization of an object from another.

The enum class introduces "strong" scoping. This ensures that the enumerator names are not directly accessible without the name of the enumeration, thus preventing name clashes and polluting the global namespace.

The mismatch algorithm in C++ STL is used to compare two ranges and returns a pair of iterators pointing to the first position where the two ranges differ.

The "extern" keyword can be used to explicitly instantiate a template. While templates are usually instantiated implicitly when they are used, in certain scenarios, developers might want to instantiate a template explicitly to ensure that instantiation happens in a particular translation unit, and "extern" facilitates this.

When inheritance in C++ is public, the protected members of the base class (Class A in this context) remain protected in the derived class (Class B). This ensures that they are accessible within Class B but not outside of it.

In C++, when defining a class template, the "template" keyword is used. This keyword precedes the class definition and indicates to the compiler that it's a template definition. When creating an instance of the class template, you'll provide specific types for the template parameters, but the definition itself begins with "template".

In C++, when a for loop does not specify initialization, condition, and increment, it becomes an infinite loop. This means it will keep running indefinitely unless an external factor (like a break statement) interrupts its execution.