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In order to make an object non-copiable we can explicitly delete both its copy-constructor and copy-assignment operator.
My question is: What is the right place to do it - in the public, private or protected section of the class? And - does this choice make any difference?
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When to delete copy constructor and assignment operator? 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 copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously. The copy constructor is used to − Initialize one object from another of the same type. Copy an object to pass it as an argument to a function.
Deleted function declaration is a new form of function declaration that is introduced into the C++11 standard. To declare a function as a deleted function, you can append the =delete; specifier to the end of that function declaration. The compiler disables the usage of a deleted function.
what is the right place to do it - in the public, private or protected section of the class?
I would put them in the public section.
This is because deleting a constructor or an assignment operator is orthogonal to making them private / protected; and when these aren't deleted, they are public by default. Putting the deletions in one of those two sections seems to me like hinting "If I hadn't deleted them, I would have made them private/protected" - which is not a message you want to convey in your case.
Note, though, that the compiler doesn't care which section you put the deletion in.
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Does where we put the deleted definition make any difference?
From a pure language standpoint it makes absolutely zero difference. Name lookup and overload resolution happen before access checking. And attempting to refer to a deleted function at the end of overload resolution makes your program ill-formed, period. A compiler may or may not issue another diagnostic about the accessibility, but the program already has an error that must be reported.
So you can put that deleted definition with whatever accessibility you desire. I think most will keep it private, to be inline with the "old" practice of making a class non-copyable (put the declaration of those members in the private section of the class, and not define them), if only to help those who know the old ways "get it" sooner. A mixture of idioms, if you would.
Marking as private is also something you can't avoid if you need to support both C++03 and C++11 mode. With the help of a macro, a header can be made to conform to both standards easily:
#if __cplusplus >= 201103L #define DELETED_DEFINITION = delete #else #define DELETED_DEFINITION #endif class noncopyable { private: // This header can be compiled as both C++11 and C++03 noncopyable(noncopyable const&) DELETED_DEFINITION; void operator=(noncopyable const&) DELETED_DEFINITION; }; If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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