Explain C++ default constructor generation rules

Q: Explain C++ default constructor generation rules

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Question

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.

Explain C++ default constructor generation rules

Solution

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