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.

Tail recursion optimization, often referred to as Tail Call Optimization (TCO), primarily serves to improve memory efficiency. When a recursive call is the last thing a function does (tail recursion), some languages can optimize it to reuse the current function's stack frame for the next function call. This can lead to significant savings in stack space, especially for deep recursive calls. It allows recursion-intensive algorithms to run without consuming as much memory or leading to a stack overflow.

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#.

Encapsulation is a key concept in object-oriented programming where the data (members) and the functions (methods) that operate on this data are bundled together as a single unit known as a class.

In C++, the 'this' pointer is an implicit pointer to the object on which a member function is called. Since friend functions are not member functions of a class, they do not have access to this pointer. They operate as external functions with special access permissions.

Implementing journaling or write-ahead logging means changes are first logged to a separate, sequential log. In case of failures, this log can be used to bring the file back to a consistent state. This provides a safeguard against data corruption or loss during unforeseen events.

The friend keyword should be used judiciously. By limiting its use only to those classes or functions that genuinely need access, you maintain a degree of encapsulation and make the design intent clear, ensuring that only necessary components have deeper access.