Is a copy constructor always created by default?

Question

As I knew until today there are four default things create when creating a new class. "Default constructor", "Destructor", "Copy constructor" and "Assignment operator". But today when I was going trough a C++ article, it said that there can be situations where the copy constructor is not create by default.

1. Is that true?
2. If it is, in which situations?
3. In those situations, how can an instance of that class be passed by value?

Answer 1

1) Yes, there can be situations where the copy constructor is not created by default.

2) The conditions where the implicitly declared default constructor is deleted are laid out in 12.8 Copying and moving class objects [class.copy]:

12.8.7 is about how the declaration of other special member functions affect the implicitly declared copy constructor. Whenever the class declares a copy constructor, a move constructor or a move assignment operator. If it declares any one of those, then you don't get an implicitly declared one.

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⁷ If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.

12.8.11 is about how the data members and base classes affect the implicitly declared copy constructor. Essentially, if the class has any data members or base classes that are not copyable, the implicitly declared copy constructor is deleted:

¹¹ An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/ move constructor for a class X is defined as deleted (8.4.3) if X has:

— a variant member with a non-trivial corresponding constructor and X is a union-like class,

— a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,

— a direct or virtual base class B that cannot be copied/moved because overload resolution (13.3), as applied to B’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,

— any direct or virtual base class or non-static data member of a type with a destructor that is deleted or inaccessible from the defaulted constructor,

— for the copy constructor, a non-static data member of rvalue reference type, or

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3) You can declare and define (either by providing an implementation or defaulting a copy constructor, or a move copy constructor, or both.