Vfrdtyky

When given code of the following structure

template <typename... Args>

void foo(Args&&... args) { ... }

I've often seen library code use static_cast<Args&&> within the function for argument forwarding. Typically, the justification for this is that using a static_cast avoids an unnecessary template instantiation.

Given the language's reference collapsing and template deduction rules. We get perfect forwarding with the static_cast<Args&&>, the proof for this claim is below (within error margins, which I am hoping an answer will enlighten)

• When given rvalue references (or for completeness - no reference qualification as in this example), this collapses the references in such a way that the result is an rvalue. The rule used is && && -> && (rule 1 above)


• When given lvalue references, this collapses the references in such a way that the result is an lvalue. The rule used here is & && -> & (rule 2 above)

This is essentially getting foo() to forward the arguments to bar() in the example above. This is the behavior you would get when using std::forward<Args> here as well.

Question - why use std::forward in these contexts at all? Does avoiding the extra instantiation justify breaking convention?

Howard Hinnant's paper n2951 specified 6 constraints under which any implementation of std::forward should behave "correctly". These were

1. Should forward an lvalue as an lvalue


2. Should forward an rvalue as an rvalue


3. Should not forward an rvalue as an lvalue


4. Should forward less cv-qualified expressions to more cv-qualified expressions


5. Should forward expressions of derived type to an accessible, unambiguous base type


6. Should not forward arbitrary type conversions

(1) and (2) were proven to work correctly with static_cast<Args&&> above. (3) - (6) don't apply here because when functions are called in a deduced context, none of these can occur.

Note: I personally prefer to use std::forward, but the justification I have is purely that I prefer to stick to convention.

forward expresses the intent and it may be safer to use than static_cast: static_cast considers conversion but some dangerous and supposedly non-intentional conversions are detected with forward:

struct A{

   A(int);

   };



template<class Arg1,class Arg2>

Arg1&& f(Arg1&& a1,Arg2&& a2){

   return static_cast<Arg1&&>(a2); //  typing error: a1=>a2

   }



template<class Arg1,class Arg2>

Arg1&& g(Arg1&& a1,Arg2&& a2){

   return forward<Arg1>(a2); //  typing error: a1=>a2

   }



void test(const A a,int i){

   const A& x = f(a,i);//dangling reference

   const A& y = g(a,i);//compilation error

  }

_{Example of error message: compiler explorer link}

How applies this justification: Typically, the justification for this is that using a static_cast avoids an unnecessary template instantiation.

Is the compilation time more problematic than code maintainability? Should the coder even lose its time considering minimizing "unnecessary template instantiation" at every line in the code?

When a template is instantiated, its instantiation causes instantiations of template that are used in its definition and declaration. So that, for example if you have a function as:

  template<class T> void foo(T i){

     foo_1(i),foo_2(i),foo_3(i);

     }

where foo_1,foo_2,foo_3 are templates, the instantiation of foo will cause 3 instantiations. Then recursively if those functions cause the instantiation of other 3 template functions, you could get 3*3=9 instantiations for example. So you can consider this chain of instantiation as a tree where a root function instantiation can cause thousands of instantiations as an exponentially growing ripple effect. On the other hand a function like forward is a leaf in this instantiation tree. So avoiding its instantiation may only avoid 1 instantiation.

So, the best way to avoid template instantiation explosion is to use dynamic polymorphism for "root" classes and type of argument of "root" functions and then use static polymorphism only for time critical functions that are virtually upper in this instantiation tree.

So, in my opinion using static_cast in place forward to avoid instantiations is a lost of time compared to the benefit of using a more expressive (and safer) code. Template instantiation explosion is more efficiently managed at code architecture level.

Scott Meyers says that std::forward and std::move are mainly for convenience. He even states that std::forward can be used to perform the functionality of both std::forward and std::move.

Some excerpts from "Effective Modern C++":

Item 23:Understand std::move and std::forward

...

The story for std::forward is similar to that for std::move, but whereas std::move unconditionally casts its argument to an rvalue, std::forward does it only under certain conditions. std::forward is a conditional cast. It casts to an rvalue only if its argument was initialized with an rvalue.

...

Given that both std::move and std::forward boil down to casts, the only difference being that std::move always casts, while std::forward only sometimes does, you might ask whether we can dispense with std::move and just use std::forward everywhere. From a purely technical perspective, the answer is yes: std::forward can do it all. std::move isn’t necessary. Of course, neither function is really necessary, because we could write casts everywhere, but I hope we agree that that would be, well, yucky.

...
std::move’s attractions are convenience, reduced likelihood of error, and greater clarity...

For those interested, comparison of std::forward<T> vs static_cast<T&&> in assembly (without any optimization) when called with lvalue and rvalue.

Here are my 2.5, not so technical cents for this: the fact that today std::forward is indeed just a plain old static_cast<T&&> does not mean that tomorrow it also will be implemented in exactly the same way. I think the committee needed something to reflect the desired behaviour of what std::forward achieves today hence, the forward which does not forward anything anywhere came into existence.

With having the required behaviour and expectations formalized under the umbrella of std::forward, just theoretically speaking, noone impedes a future implementer to provide the std::forward as not the static_cast<T&&> but something specific his own implementation, without actually taking into consideration static_cast<T&&> because the only fact that matters is the correct usage and behaviour of std::forward.