C++ Default Constructor Generation Rules

You are given a C++ class type T.

1. Under what conditions will the compiler implicitly declare a default constructor for T?
2. When will an implicitly-declared default constructor be defined as deleted, making T t; an ill-formed statement?
3. How do user-declared special member functions (copy constructor, move constructor, copy assignment operator, move assignment operator, destructor) influence the implicit generation or deletion of a default constructor?
4. Describe the impact on default constructor generation when members or base classes lack default constructors, or when members are references or const qualified.

Your answer should reference the C++ “special member functions” model (commonly known as the Rule of 0/3/5) and include small code examples illustrating classes that:

• Get an implicit default constructor.
• Do not get an implicit default constructor.
• Get an implicit default constructor that is subsequently deleted.