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No way to use C++ template with template parameters #3252

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GregoryMorse opened this issue Nov 28, 2019 · 1 comment
Open

No way to use C++ template with template parameters #3252

GregoryMorse opened this issue Nov 28, 2019 · 1 comment

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@GregoryMorse
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@GregoryMorse GregoryMorse commented Nov 28, 2019

I would like to use the function std::make_reverse_iterator:

cdef extern from "<iterator>" namespace "std" nogil:
    cdef cppclass iter_reference_t[T]:
        pass
    cdef cppclass iterator[C, T, D, P, R]:
        pass
    cdef cppclass reverse_iterator[T]:
        T iterator_type
        iter_reference_t[T]& operator*()
        reverse_iterator& operator++()
        bint operator!=(reverse_iterator)
    cdef reverse_iterator[T] make_reverse_iterator[T](T)

Gives errors such as:

while it != childs.rend(): ->
Cannot assign type 'reverse_iterator' to 'reverse_iterator[iterator]'
deref(it) ->
Cannot assign type 'iter_reference_t[iterator]' to 'Node_ptr'

My solution so far which fully compiles and runs successfully is extending reverse_iterator of the class I am using with 2 constructor definitions:

        cppclass reverse_iterator:
            reverse_iterator()
            reverse_iterator(iterator)

However, make_reverse_iterator can have some extra logic since the constructor version is not guaranteed to work and depends on implementation specific details in some cases. I am using the set class where indeed it does work at least in a Windows environment which is compiling with MSVC 2019 and its associated STL libraries.

Is there anyway to import this as the reverse_iterator must have a template class of an Iterator template class with its 5 template parameters. But I have tried many things, and I always end up getting back to the same problem that I have no way to get the reference (among other things like specific reverse_iterator not being templated in the class I am using).

Otherwise this might be a feature request as I realize especially supporting template constructs through C++20 is a pretty broad scope task.

Also separately, I see the definitions in libcpp for reverse_iterators seem to have a lot of iterators in them which could be fixed (for ++ and --). Some of them seem correct, others do not - quite inconsistent. A simple PR if this is in fact buggy which it strongly looks like e.g. for set:

        cppclass reverse_iterator:
            T& operator*()
            iterator operator++()
            iterator operator--()
            bint operator==(reverse_iterator)
            bint operator!=(reverse_iterator)
@GregoryMorse

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@GregoryMorse GregoryMorse commented Nov 28, 2019

I got it to work by being quite clever with:

cdef extern from "<iterator>" namespace "std" nogil:
    cdef cppclass reverse_iterator[T]:
        pass
    cdef reverse_iterator[T] make_reverse_iterator[T](T)

and in the set class:

        cppclass reverse_iterator:
            reverse_iterator()
            reverse_iterator(reverse_iterator)

So I can simply pass the result of make_reverse_iterator to the constructor of reverse_iterator in the class. This however requires a cast to <set[…].reverse_iterator(<set[…].reverse_iterator>object). Probably the best solution at the moment. Compiles and works though. Probably could just use a cast and not use the extra construction also. And after testing, <set[…].reverse_iterator>make_reverse_iterator(...) works fine, and is the solution then.

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