BinaryOperation boost::transform only stops when it reaches the end of rng1, even if rng2 is shorter

[David Sanders <david.sanders@…>]

Here's a complete program that reproduces the problem:

#include <boost/range/algorithm/transform.hpp> #include <boost/range/irange.hpp>

namespace std {

ostream& operator<<(ostream& stream, const pair<int, int>& p) {

return stream << "(" << p.first << ", " << p.second << ")";

}

}

int main() {

boost::transform(

boost::irange(0, 100), boost::irange(0, 10), std::ostream_iterator<std::pair<int,int>>(std::cout, "\n"), [](const int left, const int right) { return std::make_pair(left, right); });

return EXIT_SUCCESS;

}

I compile on Fedora 20 using g++ 4.8.2. Here's the output I get:

g++ transform.cpp -std=c++11 && ./a.out (0, 0) (1, 1) (2, 2) (3, 3) (4, 4) (5, 5) (6, 6) (7, 7) (8, 8) (9, 9) a.out: /usr/include/boost/range/algorithm/transform.hpp:63: OutputIterator boost::range_detail::transform_impl(SinglePassTraversalReadableIterator1, SinglePassTraversalReadableIterator1, SinglePassTraversalReadableIterator2, SinglePassTraversalReadableIterator2, OutputIterator, BinaryFunction) [with SinglePassTraversalReadableIterator1 = boost::range_detail::integer_iterator<int>; SinglePassTraversalReadableIterator2 = boost::range_detail::integer_iterator<int>; OutputIterator = std::ostream_iterator<std::pair<int, int> >; BinaryFunction = main()::lambda0]: Assertion `first2 != last2' failed. Abort

Here's the code I believe to be broken in boost/range/algorithm/transform.hpp:

template< class SinglePassTraversalReadableIterator1,

class SinglePassTraversalReadableIterator2, class OutputIterator, class BinaryFunction >

inline OutputIterator transform_impl(SinglePassTraversalReadableIterator1 first1,

SinglePassTraversalReadableIterator1 last1, SinglePassTraversalReadableIterator2 first2, SinglePassTraversalReadableIterator2 last2, OutputIterator out, BinaryFunction fn)

{

for (; first1 != last1; ++first1, ++first2) {

BOOST_ASSERT( first2 != last2 ); *out = fn(*first1, *first2); ++out;

} return out;

}

Here's my proposed fix:

template< class SinglePassTraversalReadableIterator1,

class SinglePassTraversalReadableIterator2, class OutputIterator, class BinaryFunction >

inline OutputIterator transform_impl(SinglePassTraversalReadableIterator1 first1,

SinglePassTraversalReadableIterator1 last1, SinglePassTraversalReadableIterator2 first2, SinglePassTraversalReadableIterator2 last2, OutputIterator out, BinaryFunction fn)

{

for (; first1 != last1 && first2 != last2; ++first1, ++first2) {

*out = fn(*first1, *first2); ++out;

} return out;

}

I added a check that first2 != last2 and removed the ASSERT.

Here's the relevant documentation: BinaryOperation version:

transform assigns the value z to each element [out, out + min(distance(rng1), distance(rng2))), z = fun(x,y) where x is the corresponding value in rng1 and y is the corresponding value in rng2. This version of transform stops upon reaching either the end of rng1, or the end of rng2. Hence there isn't a requirement for distance(rng1) == distance(rng2) since there is a safe guaranteed behaviour, unlike with the iterator counterpart in the standard library.

---

Please note that the original implementation doesn't meet the specification described above, whereas the new implementation does. BinaryOperation boost::for_each has a similar specification, but it implements it correctly.

Boost Rocks! Keep up the stellar work!

Don't hesitate to contact me if any of the above needs further explanation.

Thanks, David

[Neil Groves]

This is fixed on the "develop" branch.

[anonymous]

Release window for 1.56 has been extended so I can put this into 1.56.

[Neil Groves]

Merged to master and ready for the next release.