c++11 - Why can't I mix C++ universal references with standard reference? -


i'm trying understand c++11's universal references, , wrote following code:

#include <cstdio>                                                                                                                                                                                                                                              template <typename l, typename r>                                                                                                                                                                                                                             static void run(l&& lhs, const r& rhs) {                                                                                                                                                                                                                        lhs += rhs;                                                                                                                                                                                                                                                 }                                                                                                                                                                                                                                                              static void run2(int&& lhs, const int& rhs) {                                                                                                                                                                                                                   lhs += rhs;                                                                                                                                                                                                                                                 }                                                                                                                                                                                                                                                              int main() {                                                                                                                                                                                                                                                    int = 0;                                                                                                                                                                                                                                                    int b = 3;                                                                                                                                                                                                                                                    run(a, b);                                                                                                                                                                                                                                                    printf("%d\n", a);                                                                                                                                                                                                                                            // not compile.                                                                                                                                                                                                                                     run2(a, b);                                                                                                                                                                                                                                                   printf("%d\n", a);                                                                                                                                                                                                                                            return 0;                                                                                                                                                                                                                                                   }

note run() works, line calling run2() fail compile. can't figure out why. error no known conversion 'int' 'int &&' 1st argument.

i'm sure compiler right, can't figure out under why. seems run() , run2() doing same things right?

btw, changing run2() templated single parameter not work.

"universal references" can happen in contexts type of reference deduced, such in template type context t&&. in case, t can either deduced int rvalue argument, , have int&& argument type; or int& lvalue argument, give type int& && per reference collapsing rules becomes int&. in run2 function, type directly int&& , cannot bind lvalue unless use std::move.


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