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

If you want to conditionally disable your copy constructor, you definitely want it to participate in overload resolution - because you want it to be a loud compile error if you try to copy it.

And to do that, all you need is static_assert:

template 
class C {
public:
    C(const C& rhs) {
        static_assert(some_requirement_on::value, 
            "copying not supported for T");
    }
};

This will allow copy construction only if some_requirement_on is true, and if it's false, you can still use the rest of the class... just not copy construction. And if you do, you'll get a compile error pointing to this line.

Here's a simple example:

template 
struct Foo
{
    Foo() { }

    Foo(const Foo& ) {
        static_assert(std::is_integral::value, "");
    }

    void print() {
        std::cout << "Hi" << std::endl;
    }
};

int main() {
    Foo f;
    Foo g(f); // OK, satisfies our condition
    g.print();     // prints Hi

    Foo h;
    //Foo j(h); // this line will not compile
    h.print(); // prints Hi
}