Direct recursion occurs when a function calls itself directly, whereas indirect recursion involves a function calling another function that eventually calls the original function. It's a cycle of calling between two or more functions.

In a for loop, if the condition is omitted, it's treated as always true. Therefore, the loop will execute indefinitely unless there's a break statement or some external factor that interrupts its execution.

In a switch-case structure in C++, the conditions or cases must be of an integer or enumeration type. Floating-point and boolean values cannot be used as cases.

When a class contains a pointer that has memory allocated to it dynamically, it's crucial to have a destructor to release that memory when the object is destroyed to prevent memory leaks.

The continue statement, when encountered inside a loop, causes the program to skip the rest of the current iteration and move directly to the loop condition to check if the next iteration should commence. In essence, it "continues" to the next iteration of the loop without finishing the current one.

A catch block with an ellipsis (...) is used to catch all exceptions, irrespective of their type. This can be useful when you want to ensure that no exceptions go unhandled, but it's often a good practice to handle specific exceptions with their respective catch blocks.

The close function is used in conjunction with file streams in C++ to close an open file. It's crucial to close files after operations to release system resources and ensure that all changes, if any, are flushed and saved appropriately to the file system.

A destructor is a special member function of a class that is executed whenever an object of that class goes out of scope or is explicitly destroyed. Its primary role is to release any resources that the object may have acquired during its lifetime.

The "Diamond Problem" arises due to multiple inheritance when a particular class is accessible through multiple paths, often causing ambiguity. For instance, if two base classes A and B have the same method and a derived class inherits both A and B, there's ambiguity on which method to call if not overridden in the derived class.

A pure virtual function in C++ is declared using "= 0" and doesn't have an implementation in the base class. It's a way to ensure that derived classes provide their own implementation of the function, effectively making the base class abstract. This ensures a consistent interface.