FallingFromBed FallingFromBed - 1 year ago 64
C++ Question

Why adding `const` makes the universal reference as rvalue

I have been reading about the universal references in Scott's last master piece about the c++11 and 14 with that being said despite an argument assigned to either lvalue or an rvalue type reference parameter there is something in between called universal reference which could deduced to either l/rvalue based on the type trait of an argument that passed . I could understand what makes the parameter as an universal reference but the one thing that doesn't clear to me is why adding const to the type parameter

const T&& p
make the p as rvalue:

template<typename T>
void f(T&& param); // param is an universal reference

template<typename T>
void f(const T&& param); // param is an rvalue reference

Does the
do more than this when assigned to the reference parameter.

Answer Source

The official name is not universal reference, but forwarding reference. The Standard states that only rvalue references to cv-unqualified template parameters fall in this category: Deducing template arguments from a function call [temp.deduct.call]

3 If P is a cv-qualified type, the top level cv-qualifiers of P’s type are ignored for type deduction. If P is a reference type, the type referred to by P is used for type deduction. A forwarding reference is an rvalue reference to a cv-unqualified template parameter. If P is a forwarding reference and the argument is an lvalue, the type “lvalue reference to A” is used in place of A for type deduction. [ Example:

template <class T> int f(T&& heisenreference);
template <class T> int g(const T&&);
int i;
int n1 = f(i); // calls f<int&>(int&)
int n2 = f(0); // calls f<int>(int&&)
int n3 = g(i); // error: would call g<int>(const int&&), which
               // would bind an rvalue reference to an lvalue

— end example ]

Allowing const T&& to behave as forwarding references, would make it impossible to overload a template function who takes only an rvalue reference as parameter.

Update: as @HowardHinnant mentions in the comments, const T&& does have its uses (see also this Q&A).

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