The ~ operator is the bitwise NOT operator in C++. It inverts each bit of a number, changing 1s to 0s and 0s to 1s. & is the bitwise AND, | is the bitwise OR, and ^ is the bitwise XOR operator.

In C++, the "std" stands for the standard namespace. A namespace is a declarative region that provides a scope to the identifiers inside it. Using the "std" namespace, we can access features of the C++ Standard Library without prepending their names with std:: each time.

Using exception specifications like throw(type) can lead to unpredictable behavior if a different type of exception is thrown. In C++11 and later, these exception specifications are deprecated in favor of noexcept. If a function with a dynamic exception specification throws an exception not listed, it will call std::unexpected().

The standard library in C++ provides a set of predefined classes that represent standard exception types. Developers can leverage these classes to catch common errors or even derive their own custom exception types from these base classes.

The goto statement in C++ provides an unconditional jump from the goto to a labeled statement found elsewhere in the code. Although it allows for more flexibility in controlling the flow, its overuse or misuse can make the code less readable and more prone to errors. The modern programming paradigm usually discourages its use.

Abstraction in C++ refers to the concept of hiding the complicated reality of an object while only exposing operations relevant for that object. This allows for simplifying complex systems by breaking them down into smaller, more manageable parts, enabling developers to focus on high-level functionalities.

Each recursive call creates a new set of the function's local variables. This is maintained in what's called the call stack, where each call has its own memory space.

Tail-call optimization (or TCO) is a technique where the compiler replaces a function's tail call (a recursive call that's the last thing the function does) with a jump instruction, essentially converting the recursion into iteration and thereby avoiding new stack frame creation.

In C++, most of the operators can be overloaded with a few exceptions. Among the operators that cannot be overloaded are: the scope resolution operator ::, the member selection operators . and .*, and the ternary conditional operator ?:.

In C++, to declare a function that returns an integer, the correct syntax is "int functionName()". The type "int" signifies that the function's return type is an integer, and the parentheses "()" indicate it's a function declaration. Actual implementation or body of the function would follow the declaration.