Conditionally disabling a copy constructor

Question

Suppose I\'m writing a class template C that holds a T value, so C can be copyable only if T is copyabl

Answer 1

However, it's not always possible to structure your class so that defaulted constructors will do the right thing.

It's usually possible with enough effort.

Delegate the work that can't be done by a defaulted constructor to another member, or wrap the T member in some wrapper that does the copying, or move it into a base class that defines the relevant operations.

Then you can define the copy constructor as:

  C(const C&) = default;

Another way to get the compiler to decide whether the default definition should be deleted or not is via a base class:

template
struct copyable_characteristic { };

template<>
struct copyable_characteristic {
  copyable_characteristic() = default;
  copyable_characteristic(const copyable_characteristic&) = delete;
};

template 
class C
: copyable_characteristic::value>
{
 public:
  C(const C&) = default;
  C(C&& rhs);

  // other stuff
};

This can be used to delete operations using arbitrary conditions, such as is_nothrow_copy_constructible rather than just a straightforward T is copyable implies C is copyable rule.