Mixing pass-by-reference and pass-by-value to variadic template function valid?

Question

I have a method which allocates memory for a object and then calls its constructor - a memory allocator.

template <class T, typename... Arguments>
inline T* AllocateObject(Arguments... args) { return new (InternalAllocate(sizeof(T))) T(args...); }

Is it valid using this function to mix pass-by-value and pass-by-reference? For example allocating a class with a constructor with some by-value and some by-reference. It compiles, but I'm not sure if it has any nasty side-effects or not.

You should probably use `T(std::forward(args)...)`, but besides that you should be fine. — nikolas, Oct 18 '13 at 07:40

score 5 · Accepted Answer · answered Oct 18 '13 at 07:48

5

What you're looking for is perfect forwarding, ie. your AllocateObject function should be completely transparent as far as copying side effects are concerned.

This involves both std::forward (as nijansen already mentioned) and the use of universal references in your parameter list:

template <class T, typename... Arguments>
inline T* AllocateObject(Arguments&&... args)
//                                ^^ universal references
{
    return new (InternalAllocate(sizeof(T))) T(std::forward<Arguments>(args)...);
    //                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ forwarding
}

answered Oct 18 '13 at 07:48

syam

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Yeah I did indeed forget `&&`, that's kind of important for perfect forwarding ;) – nikolas Oct 18 '13 at 07:53
Sidenote: Universal References have been renamed to "Forwarding References" by now. – Jakob Riedle Aug 31 '17 at 08:20

Mixing pass-by-reference and pass-by-value to variadic template function valid?

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