Patryk Patryk - 2 months ago 7
C++ Question

How to adapt a custom container type to BOOST_FOREACH?

I would like to use

with one of legacy container type in my code base which I cannot change.

I have the following methods defined on that type:


  • .length()
    returning the current number of elements in the container

  • .operator[](unsigned i)
    returning a reference to element under index
    i



I know that I need to make my container type to satisfy Single Pass Range Concept as the boost documentation suggest but I am somehow lost since I cannot modify the type.

Any hints/solutions will be useful.

EDIT

I tried to go with suggestion but this code

struct wrapper
{
struct iterator : std::iterator<std::forward_iterator_tag, ObservationReport>
{
iterator(ObservationReportList* p, int i) : pnt_(p), i_(i) {}

ObservationReport& operator*() {
return (*pnt_)[i_];
}

iterator& operator++() {
++i_;
return *this;
}

bool operator==(const iterator& r) const {
return i_ == r.i_;
}
bool operator!=(const iterator& r) const {
return i_ != r.i_;
}

ObservationReportList* pnt_;
int i_;
};

wrapper(ObservationReportList & n)
: begin_( boost::addressof(n), 0 )
, end_( boost::addressof(n), n.length() )
{}

iterator begin() { return begin_; }
iterator end() { return end_; }

iterator begin_, end_;
};

// usage

ObservationReportList reportList;
// filling reportList
BOOST_FOREACH(ObservationReport o, wrapper(reportList))
{
}


gives me the following compiler errors:

In file included from /usr/include/boost159/boost/foreach.hpp:71:0,
from test/src/MessageFillerTest.cpp:18:
/usr/include/boost159/boost/mpl/eval_if.hpp: In instantiation of ‘struct boost::mpl::eval_if<mpl_::bool_<true>, boost::range_const_iterator<wrapper, void>, boost::range_mutable_iterator<wrapper, void> >’:
/usr/include/boost159/boost/foreach.hpp:359:13: required from ‘struct boost::foreach_detail_::foreach_iterator<wrapper, mpl_::bool_<true> >’
/usr/include/boost159/boost/foreach.hpp:679:1: required by substitution of ‘template<class T> boost::foreach_detail_::auto_any<typename boost::foreach_detail_::foreach_iterator<T, mpl_::bool_<true> >::type> boost::foreach_detail_::begin(
boost::foreach_detail_::auto_any_t, boost::foreach_detail_::type2type<T, mpl_::bool_<true> >*, bool*) [with T = wrapper]’
test/src/MessageFillerTest.cpp:206:3: required from here
/usr/include/boost159/boost/mpl/eval_if.hpp:38:31: error: no type named ‘type’ in ‘boost::mpl::eval_if<mpl_::bool_<true>, boost::range_const_iterator<wrapper, void>, boost::range_mutable_iterator<wrapper, void> >::f_ {aka struct boost::ran
ge_const_iterator<wrapper, void>}’
typedef typename f_::type type;
^
In file included from test/src/MessageFillerTest.cpp:18:0:
test/src/MessageFillerTest.cpp: In member function ‘virtual void MessageFiller_XXX_Test::TestBody()’:
/usr/include/boost159/boost/foreach.hpp:1020:39: error: no matching function for call to ‘begin(const boost::foreach_detail_::auto_any_base&, boost::foreach_detail_::type2type<wrapper, mpl_::bool_<true> >*, bool*)’
, BOOST_FOREACH_SHOULD_COPY(COL))
^
/usr/include/boost159/boost/foreach.hpp:1101:77: note: in expansion of macro ‘BOOST_FOREACH_BEGIN’
if (boost::foreach_detail_::auto_any_t BOOST_FOREACH_ID(_foreach_cur) = BOOST_FOREACH_BEGIN(COL)) {} else \
^
test/src/MessageFillerTest.cpp:206:3: note: in expansion of macro ‘BOOST_FOREACH’
BOOST_FOREACH(ObservationReport o, wrapper(reportList))
^
/usr/include/boost159/boost/foreach.hpp:660:1: note: candidate: template<class T, class C> boost::foreach_detail_::auto_any<typename boost::foreach_detail_::foreach_iterator<T, C>::type> boost::foreach_detail_::begin(boost::foreach_detail_
::auto_any_t, boost::foreach_detail_::type2type<T, C>*, mpl_::true_*)
begin(auto_any_t col, type2type<T, C> *, boost::mpl::true_ *) // rvalue
^
/usr/include/boost159/boost/foreach.hpp:660:1: note: template argument deduction/substitution failed:
/usr/include/boost159/boost/foreach.hpp:964:46: note: cannot convert ‘boost::foreach_detail_::should_copy_impl(((mpl_::bool_<false>*)0u), ((mpl_::bool_<false>*)0u), (& _foreach_is_rvalue206))’ (type ‘bool*’) to type ‘mpl_::true_* {aka mp
l_::bool_<true>*}’
(boost::foreach_detail_::should_copy_impl( \
^
/usr/include/boost159/boost/foreach.hpp:1020:9: note: in expansion of macro ‘BOOST_FOREACH_SHOULD_COPY’
, BOOST_FOREACH_SHOULD_COPY(COL))
^
/usr/include/boost159/boost/foreach.hpp:1101:77: note: in expansion of macro ‘BOOST_FOREACH_BEGIN’
if (boost::foreach_detail_::auto_any_t BOOST_FOREACH_ID(_foreach_cur) = BOOST_FOREACH_BEGIN(COL)) {} else \
^
test/src/MessageFillerTest.cpp:206:3: note: in expansion of macro ‘BOOST_FOREACH’
BOOST_FOREACH(ObservationReport o, wrapper(reportList))
^
/usr/include/boost159/boost/foreach.hpp:668:1: note: candidate: template<class T, class C> boost::foreach_detail_::auto_any<typename boost::foreach_detail_::foreach_iterator<T, C>::type> boost::foreach_detail_::begin(boost::foreach_detail_
::auto_any_t, boost::foreach_detail_::type2type<T, C>*, mpl_::false_*)
begin(auto_any_t col, type2type<T, C> *, boost::mpl::false_ *) // lvalue
^
/usr/include/boost159/boost/foreach.hpp:668:1: note: template argument deduction/substitution failed:
/usr/include/boost159/boost/foreach.hpp:964:46: note: cannot convert ‘boost::foreach_detail_::should_copy_impl(((mpl_::bool_<false>*)0u), ((mpl_::bool_<false>*)0u), (& _foreach_is_rvalue206))’ (type ‘bool*’) to type ‘mpl_::false_* {aka m
pl_::bool_<false>*}’
(boost::foreach_detail_::should_copy_impl( \
^
/usr/include/boost159/boost/foreach.hpp:1020:9: note: in expansion of macro ‘BOOST_FOREACH_SHOULD_COPY’
, BOOST_FOREACH_SHOULD_COPY(COL))
^
/usr/include/boost159/boost/foreach.hpp:1101:77: note: in expansion of macro ‘BOOST_FOREACH_BEGIN’
if (boost::foreach_detail_::auto_any_t BOOST_FOREACH_ID(_foreach_cur) = BOOST_FOREACH_BEGIN(COL)) {} else \
^
test/src/MessageFillerTest.cpp:206:3: required from here
/usr/include/boost159/boost/foreach.hpp:768:1: error: no type named ‘type’ in ‘struct boost::foreach_detail_::foreach_reference<wrapper, mpl_::bool_<true> >’

Answer

A wrapper can provide the required interface.

Note that in this example, operator[] of nasty_thing returns a reference to an int in an array of consecutive ints.

It this is not the case, we need to make the wrapper work in terms of an index.

#include <utility>
#include <memory>

//
// a nasty thing that contains consecutive ints
struct nasty_thing
{
    int& operator[](int i);
    int length();
};

// because we are dealing in consecutive ints, we can approximate an iterator with a pointer

struct wrapper
{
    using iterator = int*;

    wrapper(nasty_thing& n)
    : begin_(std::addressof(n[0]))
    , end_(begin_ + n.length())
    {}

    iterator begin() const { return begin_; }
    iterator end() const { return end_; }

    iterator begin_, end_;
};

extern nasty_thing& nt;

int main()
{
    // simulate BOOST_FOREACH
    for (auto& i : wrapper(nt))
    {

    }
}

This is the version in which nasty_thing does not represent contiguous memory:

#include <utility>
#include <memory>
#include <boost/foreach.hpp>

//
// a nasty thing that contains consecutive ints
struct nasty_thing
{
    int& operator[](int i);
    int length();
};

struct wrapper
{
    struct iterator : std::iterator<std::forward_iterator_tag, int>
    {
        iterator(nasty_thing* p, int i) : pnt_(p), i_(i) {}
        int& operator*() const {
            return (*pnt_)[i_];
        }

        iterator& operator++() {
            ++i_;
            return *this;
        }

        bool operator==(const iterator& r) const {
            return i_ == r.i_;
        }
        bool operator!=(const iterator& r) const {
            return i_ != r.i_;
        }

        nasty_thing* pnt_;
        int i_;
    };

    // needed by BOOST_FOREACH
    using const_iterator = iterator;

    wrapper(nasty_thing& n)
    : begin_{ std::addressof(n), 0 }
    , end_{std::addressof(n), n.length()}
    {}

    iterator begin() const { return begin_; }
    iterator end() const { return end_; }

    iterator begin_, end_;
};

extern nasty_thing& nt;

int main()
{
    // simulate BOOST_FOREACH
    BOOST_FOREACH(auto& i, wrapper(nt))
    {

    }
}
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