Question

This example on the usage of std::forward is puzzling me. This is my edited version:

#include <iostream>
#include <memory>
#include <utility>
using namespace std;

struct A{
    A(int&& n) { cout << "rvalue overload, n=" << n << "\n"; }
    A(int& n)  { cout << "lvalue overload, n=" << n << "\n"; }
};

template<typename> void template_type_dumper();

template<class T, class U>
unique_ptr<T> make_unique(U&& u){
    //Have a "fingerprint" of what function is being called
    static int dummyvar;
    cout<<"address of make_unique::dummyvar: "<<&dummyvar<<endl;
    //g++ dumps two warnings here, which reveal what exact type is passed as template parameter
    template_type_dumper<decltype(u)>;
    template_type_dumper<U>;

    return unique_ptr<T>(new T(forward<U>(u)));
}

int main()
{
    unique_ptr<A> p1 = make_unique<A>(2); // rvalue
    int i = 1;
    unique_ptr<A> p2 = make_unique<A>(i); // lvalue
}

The output is

address of make_unique::dummyvar: 0x6021a4
rvalue overload, n=2
address of make_unique::dummyvar: 0x6021a8
lvalue overload, n=1

and the warnings about reference to template_type_dumper show that in the first instantiation, decltype(u) = int&& and U = int, for the second decltype(u) = int& and U = int&.

It’s evident that there are two different instantiations as expected, but her are my questions:

1. how can std::forward work here? In the first instantiation, its template argument is explicitly U = int, how can it know that it has to return a rvalue-reference? What would happen if I specified U&& instead?
2. make_unique is declared to take a rvalue-reference. How come u can be a lvalue-reference? Is there any special rule that I am missing?

Answer 1

make_unique is declared to take a rvalue-reference. How come u can be a lvalue-reference? Is there any special rule that I am missing?

make_unique is declared to take a reference. What kind that reference is is to be deduced. If an lvalue of type foo is passed, U is deduced as foo& and U&& becomes foo& because of the reference collapsing rules (basically, “combining” an lvalue reference with another reference always produces an lvalue reference; combining two rvalue references produces an rvalue reference). If an rvalue of type foo is passed, U is deduced as foo and U&& is foo&&.

This is one of the things that powers perfect forwarding: with U&& you can take both lvalues and rvalues, and U is deduced to match the appropriate value category. Then with std::forward you can forward the values preserving that same value category: in the first case, you get std::forward<foo&> which forwards an lvalue, and in the second one, you get std::forward<foo> which forwards an rvalue.

In the first instantiation, its template argument is explicitly U = int, how can it know that it has to return a rvalue-reference?

Because the return type of std::forward<T> is always T&&. If you pass int it returns int&&. If you pass int& it returns int& again because of the reference collapsing rules.

What would happen if I specified U&& instead?

You would have std::forward<int&&> and the reference collapsing rules make int&& && an rvalue reference still: int&&.