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What happens (exactly) if you leave out the copy-constructor in a C++ class?
Is the class just memcpy'd or copied member wise?
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If no user-defined copy constructors are provided for a class type (struct, class, or union), the compiler will always declare a copy constructor as a non-explicit inline public member of its class.
A user-defined copy constructor is generally needed when an object owns pointers or non-shareable references, such as to a file, in which case a destructor and an assignment operator should also be written (see Rule of three).
Copy constructor (and assignment) should be defined when ever the implicitly generated one violates any class invariant. It should be defined as deleted when it cannot be written in a way that wouldn't have undesirable or surprising behaviour.
The copy constructor and copy-assignment operator are public but deleted. It is a compile-time error to define or call a deleted function. The intent is clear to anyone who understands =default and =delete . You don't have to understand the rules for automatic generation of special member functions.
The class is copied member-wise.
This means the copy constructors of all members are called.
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The default copy constructor is made mebmber wise.
The same member-wise approach is also used for creating an assignment operator; the assignment operator however is not created if the class has reference members (because it's impossible to rebind a reference after construction).
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