Avoiding virtual method in template subclass?

I want to make a template that subclasses a parent but changes just one method.

This should be easy, technically speaking, since it should all be generated code. Virtual method is not necessary, but how do I tell the compiler to see the new method?

So for example:

Code:

---


        template <typename type> class CalcA
        {
        protected:
                long calculate();
        };       

template <typename type> class CalcB: public CalcA<type>
        {
        protected:
                long calculate();
        };

---

It's forcing me to use virtual methods, but there has to be some way around this. Anyone know?

You can use the Curiously Recurring Template Pattern to call a member function of the derived class without using virtual functions.

Thank you, I think that will help.

Quote:

---

Originally Posted by originalfamousjohnsmith

It's forcing me to use virtual methods, but there has to be some way around this. Anyone know?

---

Why do you want to avoid virtual functions?

Because of performance and about 2 million other reasons. I don't mind virtual methods but it would be a big mistake here.

Not sure there are any better options for me, though. The suggested solution is a possibility but it's a little complex. Like I said it's two one line methods I want to change, but if I did it that way I'd need to change every place I call those two methods and doing that everywhere will make an unreadable mess.

I may as well spell out the whole problem. I have a vector class, and I want to have it so that it can either use the default allocation or it can use a different allocation method for when it's being used as a system object. However, I don't want things to get too complex, and I don't want to add in extra performance penalties, and I especially don't want its usage to change (ie extra unwieldly syntax every time you create something).

I guess the options are:
1. Preprocessor. Kind of ugly, but I can generate out a duplicate with small changes using preprocessor.
2. File generation. I hate doing this, but it does work.
3. Allocator helper object. Then I have to pass one in somehow, and I just transfer the problem of virtualness. Virtual inline static methods are almost guaranteed to make the compiler go crazy. Or else I have a few static concrete allocator objects, which I have to ensure get initialized before anything else.
4. function pointers. No inlining for certain, then, but at least will work.
5. CRT - kind of like 3, I guess.
6. boolean? I can't use a switch it boils down to same problem with needing to be virtual, but I can stick a boolean in, though it increases class size.
7. something else?

Quote:

---

Originally Posted by originalfamousjohnsmith

I have a vector class, and I want to have it so that it can either use the default allocation or it can use a different allocation method for when it's being used as a system object.

---

Must this decision be made at run time, or can it always be made at compile time?

Quote:

---

Originally Posted by originalfamousjohnsmith

7. something else?

---

Can't you use templates to make a policy out of the allocation strategy, like

Code:

---

template <typename AllocPolicy>
class CalcA {
public:
        void calculate() {
                AllocPolicy::calculate();
        }
};

---

Ok, yes I guess it's basically a 'policy'. Some of this does not gel well in my brain, but I see how helpful a policy can be now and what it means, and it is sort of like the CRT.

Anyway I end up with code like:

Code:

---

        template <typename type, class AllocationClass=SStandardAllocation> class Array: public SBaseArray, public AllocationClass
        {
        protected:
                type *contents;
                long count;
                long contiguousSize;
                long totalSize;
                SSimpleChunk* firstChunk;

                inline void* allocate(size_t size)
                {
                        return AllocationClass::allocate(size);
                }
                inline void deallocate(void* address, size_t size)
                {
                        AllocationClass::deallocate(address, size);
                }

---

Quote:

---

Originally Posted by originalfamousjohnsmith

Some of this does not gel well in my brain,

---

Well, if you want polymorphism there's either object orientation or generics. If you rule out virtual methods there's only templates left.

How CRTP help in this case ?

Quote:

---

Originally Posted by Peter_APIIT

How CRTP help in this case ?

---

Read the link I provided. It explains how you can simulate virtual functions without the runtime overhead using CRTP.