I can't do this:
int &&q = 7;
int &&r = q;
//Error Message:
//cannot convert from 'int' to 'int &&'
//You cannot bind an lvalue to an rvalue reference
If I understand correctly, when initializing an rvalue reference, there's a temporary variable got initialized too. So int &&q = 7;
can be considered as:
int temp = 7;
int &&q = temp;
And when using a reference on the right side, I am actually using the referee. So int &&r = q;
can be considered as:
int &&r = temp; //bind an lvalue to an rvalue reference, cause error, understandable
So above is how I understand the compiler error occurs.
Why adding std::forward
can solve that?
int &&q = 7;
int &&r = std::forward<int>(q);
I know the std::forward
always returns an rvalue reference, how is the reference returned by std::forward
different from int&&q
?
how is the reference returned by
std::forward
different fromint&&q
?
Their value categories are different. And note that types and value categories are different things.
q
is a named variable, it's qualified as lvalue , so it can't be bound to rvalue reference.
(emphasis mine)
the name of a variable, a function, a template parameter object (since C++20), or a data member, regardless of type, such as
std::cin
orstd::endl
. Even if the variable's type is rvalue reference, the expression consisting of its name is an lvalue expression;
While rvalue reference returned from function is qualified as xvalue , which belongs to rvalue .
a function call or an overloaded operator expression, whose return type is rvalue reference to object, such as
std::move(x)
;
The difference between the expressions q
and std::forward<int>(q)
is that the former is an lvalue, while the latter is an rvalue (of fundamental category xvalue).
I've addressed similar concerns in this answer : the point is that q
as an expression is an lvalue , because it has a name. std::forward<int>(q)
(or the equivalent std::move(q)
) are expressions which don't have names, and since they return (unnamed) rvalue references, they are xvalues, which is a subcategory of rvalue and can thus bind to an rvalue reference.
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