Re: [Boost-users] Implementing python's enumerate with iterator adaptors

On Tue, Apr 17, 2012 at 5:03 AM, Jens Auer wrote:

Hi,****

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I am implementing something similar to python’s enumerate using boost iterator adapter, but I am running into two problems I am unable to solve. Both problems use my Enumerate-adaptor defined as:****

#include <utility>****

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#include ****

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namespace Iterator****

{****

// some simple wrapper to have better names than std::pair****

template****

struct Enumerated****

{****

Enumerated(T i, X x):****

count(i),****

value(x)****

{****

** **

}****

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T count;****

X value;****

};****

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template< typename Iterator, typename T = typenameIterator::difference_type>

****

class Enumerator:****

public boost::iterator_adaptor,****

Iterator,****

Enumerated, ****

boost::use_default,****

Enumerated > ****

{****

public:****

Enumerator():****

mCount(),****

mPos(),****

mStep()****

{****

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}****

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template<typename It>****

explicit Enumerator(It i, T start=T(), T step=T(1) ):****

mCount(start),****

mPos(i),****

mStep(step)****

{****

** **

}****

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private:****

friend class boost::iterator_core_access;****

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Enumerated dereference() const****

{****

return Enumerated(mCount, *mPos);* ***

}****

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Enumerated dereference()****

{****

return Enumerated(mCount, mPos);** **

}

Pretty sure this latter overload of dereference will never get called, and it wouldn't compile anyway ("mPos" should be "*mPos"). ****

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bool equal(Enumerator const& other) const****

{****

return mPos == mPos;****

}

Error already noted here.

****

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void increment()****

{****

mCount += mStep;****

mPos += 1;****

}

Consider ++mPos?

****

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void decrement()****

{****

mCount -= mStep;****

mPos -= 1;****

}

Similarly, consider --mPos?

****

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void advance(typename Iterator::difference_type step)****

{****

mCount += step * mStep;****

mPos += step;****

}****

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typename Iterator::difference_type distance_to(Enumerator const& other) const****

{****

return std::distance(mPos, other.mPos);****

}****

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T mCount;****

Iterator mPos;****

typename Iterator::difference_type mStep;****

};****

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template****

Enumerator enumerate(Iterator i, T start=0, T step=1)****

{****

return Enumerator(i, start, step);****

}****

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When I compile the following sample program, msvc (VS2010) prints an error, but g++ does not have any problems.****

#include <vector>****

#include <iostream>****

#include <algorithm>****

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void foo()****

{****

std::vector<int> v;****

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auto i = Iterator::enumerate(v.begin(), 0, 1);****

auto j = Iterator::enumerate(v.end(), 0, 1);****

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std::for_each( i, j, [](decltype(*i) e)****

{****

std::cout << e.count << ", " << e.value << std::endl;****

});****

}****

Error 1 error C2665: 'std::_Debug_range2' : none of the 2 overloads could convert all the argument types d:\programme\microsoft visual studio 10.0\vc\include\xutility 728

Can you give more context to the error message?

****

My second problem can be reproduced with the following sample program on both msvc and g++:****

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#include <vector>****

#include <iostream>****

#include <algorithm>****

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void foo()****

{****

std::vector<int> v;****

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auto i = Iterator::enumerate(v.begin(), 0, 1);****

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auto y = i->count;****

auto z = i->value;****

}****

MSVC:****

Error 3 error C2664: 'boost::implicit_cast' : cannot convert parameter 1 from 'Iterator::Enumerated *' to 'boost::detail::operator_arrow_proxy<T>' c:\software development\externals\include\boost\iterator\iterator_facade.hpp 326

As Mateusz has noted, I believe you're running into https://svn.boost.org/trac/boost/ticket/5697 which was just recently fixed in trunk. See if that resolves your problem. [snip compiler trace] - Jeff