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

A noteworthy approach is partial specialization of the surrounding class template.

template ::value>
struct Foo
{
    T t;

    Foo() { /* ... */ }
    Foo(Foo const& other) : t(other.t) { /* ... */ }
};

template 
struct Foo : Foo
{
    using Foo::Foo;

    // Now delete the copy constructor for this specialization:
    Foo(Foo const&) = delete;

    // These definitions adapt to what is provided in Foo:
    Foo(Foo&&) = default;
    Foo& operator=(Foo&&) = default;
    Foo& operator=(Foo const&) = default;
};

This way the trait is_copy_constructible is satisfied exactly where T is_copy_constructible.