Declaring a function as a friend within a class gives that function the ability to access the class's private and protected members. However, it doesn't make that function a member of the class. A friend function remains a non-member function, but it's just granted special access privileges.

To determine if a year is a leap year, we need to check if the year is divisible by certain numbers. The modulus operator (%) gives the remainder of a division and can be used to check for divisibility. The equality operator (==) is used to check if the remainder is zero. Hence, the combination of % and == is crucial to check the divisibility conditions required to determine a leap year.

In tail recursion, the function's recursive call is the last action executed. The significance of tail recursion is that the current function frame can be discarded before the recursive call is made, potentially allowing compilers to optimize the recursion by reusing the current function's stack frame for the next.

A stack overflow error typically indicates that there's too much recursion, causing the system's stack to exceed its capacity. One effective way to resolve this is by converting the recursive algorithm to an iterative process using loops, which can help manage the stack better and prevent such errors. This retains the logic without heavily depending on the system's stack.

In C++, the catch keyword is used to define a block of statements that will be executed when a particular exception is thrown. This block is used to handle the exception and provide a response, ensuring that the program can continue or terminate gracefully.

An abstract class in C++ can have both abstract methods (methods without any definition) and concrete methods (methods with a definition). This allows for a mix of defined and undefined behaviors in subclasses.

Multiple inheritance in C++ allows a derived class to inherit attributes and methods from more than one base class. This contrasts with single inheritance where a class can inherit only from one superclass. C++ supports multiple inheritance, unlike some other OO languages.

The swap function in C++ STL allows for a quick swap of elements between two containers. This operation is usually done in constant time as only the internal pointers are exchanged.

In C++, when multiple 'if' conditions are true, only the block corresponding to the first true condition will be executed. Subsequent true conditions are ignored.

The break statement is primarily used for exiting a loop or a switch when a specific condition is met. In a professional codebase, it's particularly useful for scenarios where processing should not continue once a certain condition has been satisfied, ensuring efficient and intended behavior.