How to properly use references with variadic templates

问题

I have something like the following code:

   template<typename T1, typename T2, typename T3, typename T4>
   void inc(T1& t1, T2& t2, T3& t3, T4& t4) { ++t1; ++t2; ++t3; ++t4; }

   template<typename T1, typename T2, typename T3>
   void inc(T1& t1, T2& t2, T3& t3) { ++t1; ++t2; ++t3; }

   template<typename T1, typename T2>
   void inc(T1& t1, T2& t2) { ++t1; ++t2; }

   template<typename T1>
   void inc(T1& t1) { ++t1; }

I'd like to reimplement it using the proposed variadic templates from the upcoming standard. However all the examples I've seen so far online seem to be printf like examples, the difference here seems to be the use of references. I've come up with the following:

inline void inc() { }

template<typename T>
inline void inc(T&& t) { ++t; }

template<typename T,typename ... Args>
inline void inc(T&& t, Args&& ... args) { ++t; inc(args...); }

What I'd like to know is:

Should I be using r-values instead of references?
Possible hints or clues as to how to accomplish what I want correctly.
What guarantees does the new proposed standard provide in regard to the issue of the recursive function calls, is there some indication that the above variadic version will be as optimal as the original? (should I add inline or some-such?)

回答1:

I would not use rvalue references here, because that will allow you to bind to rvalues which can allow such nonsensical code as:

inc(1);

So, I would stick with regular references:

template<typename T>
void inc(T& t) { ++t; }

template<typename T,typename ... Args>
void inc(T& t, Args& ... args) { ++t; inc(args...); }

回答2:

Should I be using r-values instead of references?

You mean rvalue references? No, I see no reason for those.

Possible hints or clues as to how to accomplish what I want correctly.

You're already there. Your code should do what you want.

What guarantees does the new proposed standard provide wrt the issue of the recursive function calls, is there some indication that the above variadic version will be as optimal as the original? (should I add inline or some-such?)

The C++ standard doesn't guarantee any inlining. You could check out what the compiler generates. If you want everything to be inlined -- including the upmost inc-call -- you could put an inline to both of the functions as a request. If you want something like your non-variadic template, you could wrap it like this:

inline void inc_impl() {}

template<typename T, typename...U>
inline void inc_impl(T& t, U&...u) { ++t; inc_impl(u...); }

template<typename...T>
void inc(T&...t) { inc_impl(t...); }

Now inc is not inline while each of its implementations will probably contain no real function calls when the inlining of inc_impl calls are done -- but again, there's no guarantee.

回答3:

What guarantees does the new proposed standard provide wrt the issue of the recursive function calls, is there some indication that the above variadic version will be as optimal as the original? (should I add inline or some-such?)

The standard won't guarantee an optimization will be performed, it only specifies the behavior and the result. Whether the function will be inlined problem of the implementation.

In fact, the inline keyword is only a hint to the compiler which is often ignored because the compiler can decide better.

Finally, g++-4.5 completely inlined all inc functions at -O2. Don't know if it's what you want.

来源：https://stackoverflow.com/questions/2616082/how-to-properly-use-references-with-variadic-templates

标签

c++

c++11

templates

variadic