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

This is a bit of a trick, but it works.

template
struct block_if_helper{
  using type=T;
};
template
struct block_if_helper{
  class type{
    type()=delete;
  };
};
template
using block_if=typename block_if_helper::type;
template
using block_unless=typename block_if_helper::type;

now we create a method that is your copy ctor ... maybe.

template
struct example {
  enum { can_copy = std::is_same{} };

  example( block_unlessconst& o ); // implement this as if `o` was an `example`
  // = default not allowed
  example( block_ifconst& )=delete;
};

and now the =default is the copy ctor if and only if can_copy, and the =delete of not. The stub type that it is otherwise cannot be created.

I find this technique useful for general method disabling on compilers that do not support the default template argument feature, or for methods (like virtual or special) that cannot be templates.