In C++, parentheses have the highest precedence among the listed options. They can be used to group operations and force a desired order of evaluation. This is a fundamental concept in most programming languages to control the order in which expressions are evaluated.

When passing by reference in C++, no additional memory is allocated. Instead, a reference serves as an alias to the original variable. It simply refers to the memory location of the original variable and doesn't create a separate copy of it. This contrasts with pass by value, where a copy of the object is made, which might lead to additional memory consumption.

Functions in C++ allow code to be modularized, providing a way to encapsulate and reuse code. Their primary role is to perform a specific task or computation, ensuring the code is organized, clear, and maintainable.

When a floating-point number is converted to an integer in C++, any fractional part is discarded (i.e., truncated). This does not involve rounding, just a simple removal of the decimal and beyond.

While break and continue have their uses, over-reliance on them can make code harder to read and debug. Instead, it's often recommended to use clear and structured logic that doesn't rely on abrupt jumps within loops.

The friendship in C++ is one-way. If class A declares class B or a function as its friend, it doesn't mean that class A automatically becomes a friend of class B or that function. Friendship must be explicitly granted. This means the granting of friendship is not reciprocal by default. Class B or the function would need to separately declare class A as a friend for the reverse to be true.

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.