Re: Variadic templates and passing by reference
On 11 Mrz., 08:01, Thomas Richter wrote:
how would you realize a variadic template function where you want to
pass in both scalar types and references you want to modify in the
callee. To be specific, consider declarations as follows:
class A;
template<class ...U>
void Launch(A &t,U&... other);
template<class ...U>
void Launch(int x,U&... other);
Now I get a problem because, when I want to call "Launch" with int
arguments as in
class A a,b;
Launch(a,5,b);
it clearly doesn't work. This would require the compiler to create a
reference to a temporary, which is of course not possible. Replacing the
arguments by const references would solve the problem as it would allow
the compiler to pass in references to temporaries. However, this
prohibits of course modification of the classes passed in, which I would
like to do. Replacing the reference by a copy would of course also solve
the problem of argument passing, but then I only modify the copy, so
this doesn't help either.
Any other ideas?
Yes, write "U&&..." instead of "U&...". A reference parameter of type
"T&&" where T is deduced by the compiler will bind to anything,
rvalues _and_ lvalues because in the lvalue case T will be deduced to
be an lvalue reference and reference collapsing makes T&& in
_this_case_ an lvalue reference as well.
Depending on what you plan to do exactly in the Launch functions, the
expansion pattern "std::forward<U>(other)..." might come in handy as
it is able to restore the value category (lvalue vs rvalue) of the
passed arguments. This is called "perfect forwarding".
Cheers!
SG
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