Actually the question is how to achieve it? The use of boost::any_range is possible but undesirable (performance considerations)

Sample code:

typedef boost::multi_index_container<

int, bmi::indexed_by<bmi::hashed_unique<bmi::tag<struct someTag>,

bmi::identity<int>>>> Datum;

class bar {

public:

auto foo() const;

private:

Datum m_someDataContainer;

};

SomeType bar::foo() const {

SomeType retVal;

for (auto i = 0u; i < 10; ++i) {

retVal = boost::range::join(retVal, m_someDataContainer.equal_range(i));

}

return retVal;

}

Here  we can observe several problems. The return type cannot be easily deduced. The join would create joined_range<joined_range<joined_range and so on of nested types. The above can be achieved by using any_range like

typedef boost::any_range<int, boost::random_access_traversal_tag, int&, std::ptrdiff_t> VersionRange;

but above will create nested joined_range of any_range which, when the number of iterations is big enough, has tremendous impact on performance.

So, what can be done here?