Explain C++ default constructor generation rules
Quick Overview
This question evaluates understanding of C++ special-member-function rules—specifically when the compiler implicitly declares a default constructor, when that implicitly-declared constructor is defined as deleted, and how user-declared copy/move constructors, copy/move assignments, destructors, and member/base types (const, reference, or non-default-constructible) affect generation. Commonly asked to assess mastery of language semantics and object-initialization pitfalls, it belongs to the Software Engineering Fundamentals domain and tests both conceptual understanding of compiler behavior and practical application in C++ code.
You are given a C++ class type T.
-
When will the compiler
implicitly declare
a default constructor for
T? -
When will the implicitly-declared default constructor be
defined as deleted
(i.e.,
T t;is ill-formed)? - How do user-declared special member functions (copy/move ctor, copy/move assignment, destructor) affect whether a default constructor is generated?
-
What changes if members/base classes have no default constructors, are references, or are
const?
Answer in terms of the C++ “special member functions” model (rule of 0/3/5) and include a few small examples of classes that:
- get an implicit default constructor,
- do not get one,
- get one but it is deleted.