On 02/13/2016 03:14 PM, John Ralls wrote:
>
>> On Feb 13, 2016, at 12:25 PM, Larry Evans <cppljev...@suddenlink.net> wrote:
>>
>>
>>
>> On 02/13/2016 01:03 PM, John Ralls wrote:
>> [snip]
>>> You started out trying to encapsulate an object and a flag that
>> indicated where to put it. That had genericity problems because there's
>> no type-safe way to generically handle the differing return types.
>> Polymorphism to the rescue! Except that it turns out that you have
>> different handling requirements depending not on the type that you're
>> storing but on the actual property that you're processing.
>>>
>>> How about this:
>>> template <typename T, GncTransPropType p> class GncTransPropImpl {
>>> public:
>>> GncTransPropImpl (T v) : value {v} {}
>>> ...
>>> friend template<>void
>> set_property(std::unique_ptr<GncTransPropImpl<T, P>> prop, Transaction
>> *txn);
>>> friend template<>void
>> set_property(std::unique_ptr<GncTransPropImpl<T, P>> prop, Split *split);
>>> private:
>>> T value;
>>> }
>>
>> Why not make T a metafunction of p? IOW, define:
>>
>> template<GncTransPropType Index>
>> struct PropIndex2PropType
>> ;
>>
>> template<>
>> struct PropIndex2PropType<ACCOUNT>
>> {
>> using Type = Account*;
>> };
>> template <GncTransPropType p> class GncTransPropImpl {
>> public:
>> using T = typename PropIndexPropType::type;
>> GncTransPropImpl (T v) : value {v} {}
>> ...
>> private:
>> T value;
>> };
>>
>> That way one template argument is avoided.
>> You could also just define a typelist using
>> maybe boost::mpl::vector<T0,T1,T2,...>
>> and index into that instead of using PropIndex2PropType.
>>
>
> Nice. I haven't seen that use of 'typename' before, but I'm still a beginner
> at metaprogramming. What does it actually do there? IOW, what's the
> difference between 'using T = PropIndex2PropType::Type' and 'using T =
> typename PropIndex2PropType::Type'?
>
Without it you get a compile error. IIUC, It's only needed
within templates.
When I compile the attached, which doesn't use typename,
I get the error:
ident_meta.cpp:9:13: error: need ‘typename’ before ‘ident1<T>::type’
because ‘ident1<T>’ is a dependent scope
using T = ident1<T>::type;
^
HTH.
-rega
> Using boost::mpl::vector could also replace the abstract superclass at the
> expense of having to explicitly list all of the GncTransPropImpl
> specializations in its declaration.
>
> Aside: I'm a bit leery of using too much metaprogramming in GnuCash. While
> the gurus (Sutter, Myers, et. al.) have been promoting it for the last
> several years it seems to be pretty esoteric still and it might be too
> incomprehensible to most potential contributors.
>
I understand. I still have difficulty with metaprogramming.
> Regards,
> John Ralls
>
template<typename T>
struct ident1
{
using type=T;
};
template<typename T>
struct ident2
{
using T = ident1<T>::type;
};
int main()
{
ident2<int>::type i=0;
return i;
}
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