Chapter 101: Perfect Forwarding

Section 101.1: Factory functions

Suppose we want to write a factory function that accepts an arbitrary list of arguments and passes those arguments unmodified to another function. An example of such a function is make_unique, which is used to safely construct a new instance of T and return a unique_ptr<T> that owns the instance.

The language rules regarding variadic templates and rvalue references allows us to write such a function.

template<class T, class... A> unique_ptr<T> make_unique(A&&... args)

{

return unique_ptr<T>(new T(std::forward<A>(args)...));

}

The use of ellipses ... indicate a parameter pack, which represents an arbitrary number of types. The compiler will expand this parameter pack to the correct number of arguments at the call site. These arguments are then passed to T's constructor using std::forward. This function is required to preserve the ref-qualifiers of the arguments.

auto p2 = make_unique<foo>(std::move(f)); // calls foo::foo(foo&&) auto p3 = make_unique<foo>(1, 2, 3);