Custom memory allocator design

Question

I have memory allocator which allocates memory for an object and calls its constructor with any given arguments, see below.

 // 0 args to constructor template <class T> inline T* AllocateObject() { return new (InternalAllocate(sizeof(T), alignof(T))) T(); } // 1 args to constructor template <class T, typename arg0> inline T* AllocateObject(const arg0& a0) { return new (InternalAllocate(sizeof(T), alignof(T))) T(a0); } template <class T, typename arg0> inline T* AllocateObject(arg0& a0) { return new (InternalAllocate(sizeof(T), alignof(T))) T(a0); } // 2 args to constructor template <class T, typename arg0, typename arg1> inline T* AllocateObject(arg0& a0, arg1& a1) { return new (InternalAllocate(sizeof(T), alignof(T))) T(a0,a1); } template <class T, typename arg0, typename arg1> inline T* AllocateObject(const arg0& a0, arg1& a1) { return new (InternalAllocate(sizeof(T), alignof(T))) T(a0,a1); } template <class T, typename arg0, typename arg1> inline T* AllocateObject(arg0& a0, const arg1& a1) { return new (InternalAllocate(sizeof(T), alignof(T))) T(a0,a1); } template <class T, typename arg0, typename arg1> inline T* AllocateObject(const arg0& a0, const arg1& a1) { return new (InternalAllocate(sizeof(T), alignof(T))) T(a0,a1); } //.........

As you can see the number of calls grows with quite rapidly with the number of arguments. I have to alternate with 'const' and 'non-const' for each argument to make sure it plays fine with any argument I pass. (specifically, to be able to pass by references aswell as pass by value)

Is there any better way to perform the same task than to repeat this scheme? Basically I am looking at something like 8-10 arguments max and its just not very feasible I feel.

Thanks

If you never change the arguments, just use const type & to handle all 4 versions at once. — Qaz
– Qaz, Commented Aug 30, 2012 at 17:53
Is C++11 an option for you? It contains updates to the language very specific for this exact problem. EDIT: Well you're using alignof so shame on me for not reading more carefully. :) FireRain's answer is what I was alluding to. — GManNickG
– GManNickG, Commented Aug 30, 2012 at 17:53
also - how would that work if the c onstructor of the object might change the argument? — KaiserJohaan
– KaiserJohaan, Commented Aug 30, 2012 at 19:07

FireRain · Accepted Answer · 2012-08-30 17:53:20Z

4

You can use a variadic template.

template <class T, class... Args> inline T* AllocateObject(Args&&... args) { return new (InternalAllocate(sizeof(T), alignof(T))) T(std::forward<Args>(args)...); }

The std::forward call will preserve any references and constness.

Note that this requires C++11. Most recent compilers already support variadic templates (I am unsure about Microsoft’s, though).

answered Aug 30, 2012 at 17:53

FireRain

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10 Comments

Qaz Over a year ago

And if you need to enforce a maximum number of arguments, static_cast combined with sizeof args... will do the trick nicely.

Bob Fincheimer Over a year ago

Too bad there really isn't any compiler that really supports it yet. (a very new version of gcc does a bit I think)

Qaz Over a year ago

@BobFincheimer, I've been using variadic templates on GCC for a while now. Wasn't it way back at 4.6.1?

Mr.Anubis Over a year ago

@BobFincheimer clang and gcc supports if fully

FireRain Over a year ago

@BobFincheimer GCC and clang support variadic templates. I am only unsure about MSVC, which did have trouble with it. That may be fixed by now, but I don’t use Windows.

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Jens Agby · Accepted Answer · 2012-08-30 18:24:52Z

Not a template solution, but a variable arguments #define could help you out of this problem.
The exact format depends on your compiler, but in MSVC it would look like this:

#define ALLOCATE_OBJECT(TYPE, ...) \ ( new( InternalAllocate(sizeof(TYPE), alignof(TYPE)) ) TYPE(__VA_ARGS__) )

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