Deleting a memory location more than once leads to undefined behavior, most commonly resulting in program crashes. Always ensure memory is deleted once and pointers are set to nullptr afterwards.

The C++98 standard was officially published in 1998. It was the first standardized version of the C++ language, providing a foundation for many modern C++ features and establishing a baseline for future improvements.

A friend class in C++ is a class that is given permission to access the private and protected members of another class. To declare a class as a friend of another class, the friend keyword is used before the class declaration in the original class.

The getline function is used in C++ to read an entire line from an input stream, such as a file. This is particularly useful when dealing with strings that have spaces or when reading full sentences.

A friend function, although not a member of a class, can access its private and protected members. This provides an external function the capability to interact closely with the internals of the class.

The STL vector maintains its elements in a contiguous block of memory. Inserting or erasing elements in the middle requires shifting elements, which can be expensive, especially for large datasets. This might cause frequent reallocations and data movements, impacting performance.

The switch-case structure in C++ is best suited for scenarios where a single variable's value needs to be checked against a series of constants, often seen with enumerated values. It's a cleaner and more readable alternative to a long series of if-else statements in such cases.

Function call overhead is the extra time and resources needed when a function is called. Using inline functions can help reduce this overhead, especially in recursive algorithms, because the function's code gets inserted (or "inlined") at each point where the function is called, eliminating the need for the typical call and return process, which can be resource-intensive.

In C++, the modulo operator % is used to determine the remainder of a division operation between two numbers. For example, 7 % 3 would yield a result of 1, as when 7 is divided by 3, the remainder is 1. It's a very useful operator, especially in situations that require calculations around loops and array indices.

Abstraction involves isolating the complex reality while exposing only the necessary parts. In the context of object-oriented programming, this means defining clear interfaces for classes, which allows interactions based on these interfaces, hiding the internal complex workings. This makes the software design more understandable and manageable.