Invoking Template Function using Random Access Iterator - c++

Question

As part of an MOC, I am implementing the merge_sort subroutine, and just for curiosity's sake, I would like to implement it using a function signature similar to what std::sort does.
The code I have so far is as follows.
#include <iostream>
#include <vector>
template <typename T>
bool read_number_list(const std::string& filename, std::vector<T>& output_array){
// ... read number list into an array
return true;
}
template<typename RandomAccessIterator>
void merge_sort_array (RandomAccessIterator first, RandomAccessIterator last, std::vector<decltype(*first)>& merge_array){
// ... merge routine
}
template<typename RandomAccessIterator>
void sort_array (RandomAccessIterator first, RandomAccessIterator last){
std::vector<decltype(*first)> merge_array;
merge_array.assign(first, last);
merge_sort_array(first, last, merge_array);
}
int main(){
std::vector<int> number_array;
read_number_list<int>("file.txt", number_array);
sort_array(number_array.begin(), number_array.end());
return 0;
}
I am seeing errors in the step where I make a copy of the array to be sorted, and also at the point where I invoke the merge_sort_array method. As is customary in templated code, I get the following plethora of errors (which I find quite inscrutable):
In file included from /usr/include/x86_64-linux-gnu/c++/4.8/bits/c++allocator.h:33:0,
from /usr/include/c++/4.8/bits/allocator.h:46,
from /usr/include/c++/4.8/string:41,
from /usr/include/c++/4.8/random:41,
from /usr/include/c++/4.8/bits/stl_algo.h:65,
from /usr/include/c++/4.8/algorithm:62,
from /home/balajeerc/Projects/algorithms/src/main/cpp/lib/algorithms/sorting/merge_sorter.h:8,
from /home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:1:
/usr/include/c++/4.8/ext/new_allocator.h: In instantiation of 'class __gnu_cxx::new_allocator<int&>':
/usr/include/c++/4.8/bits/allocator.h:92:11: required from 'class std::allocator<int&>'
/usr/include/c++/4.8/bits/alloc_traits.h:90:43: required from 'struct std::allocator_traits<std::allocator<int&> >'
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: required from 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/ext/new_allocator.h:63:26: error: forming pointer to reference type 'int&'
typedef _Tp* pointer;
^
/usr/include/c++/4.8/ext/new_allocator.h:64:26: error: forming pointer to reference type 'int&'
typedef const _Tp* const_pointer;
^
In file included from /usr/include/c++/4.8/string:41:0,
from /usr/include/c++/4.8/random:41,
from /usr/include/c++/4.8/bits/stl_algo.h:65,
from /usr/include/c++/4.8/algorithm:62,
from /home/balajeerc/Projects/algorithms/src/main/cpp/lib/algorithms/sorting/merge_sorter.h:8,
from /home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:1:
/usr/include/c++/4.8/bits/allocator.h: In instantiation of 'class std::allocator<int&>':
/usr/include/c++/4.8/bits/alloc_traits.h:90:43: required from 'struct std::allocator_traits<std::allocator<int&> >'
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: required from 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/bits/allocator.h:97:26: error: forming pointer to reference type 'int&'
typedef _Tp* pointer;
^
/usr/include/c++/4.8/bits/allocator.h:98:26: error: forming pointer to reference type 'int&'
typedef const _Tp* const_pointer;
^
In file included from /usr/include/c++/4.8/ext/alloc_traits.h:36:0,
from /usr/include/c++/4.8/bits/stl_construct.h:61,
from /usr/include/c++/4.8/bits/stl_tempbuf.h:60,
from /usr/include/c++/4.8/bits/stl_algo.h:62,
from /usr/include/c++/4.8/algorithm:62,
from /home/balajeerc/Projects/algorithms/src/main/cpp/lib/algorithms/sorting/merge_sorter.h:8,
from /home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:1:
/usr/include/c++/4.8/bits/alloc_traits.h: In instantiation of 'struct std::allocator_traits<std::allocator<int&> >':
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: required from 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/bits/alloc_traits.h:100:1: error: forming pointer to reference type 'std::allocator_traits<std::allocator<int&> >::value_type {aka int&}'
_GLIBCXX_ALLOC_TR_NESTED_TYPE(pointer, value_type*)
^
/usr/include/c++/4.8/bits/alloc_traits.h:100:1: error: no matching function for call to 'std::allocator_traits<std::allocator<int&> >::_S_pointer_helper(std::allocator<int&>*)'
_GLIBCXX_ALLOC_TR_NESTED_TYPE(pointer, value_type*)
^
/usr/include/c++/4.8/bits/alloc_traits.h:100:1: note: candidate is:
/usr/include/c++/4.8/bits/alloc_traits.h:100:1: note: template<class _Tp> static typename _Tp::pointer std::allocator_traits<_Alloc>::_S_pointer_helper(_Tp*) [with _Tp = _Tp; _Alloc = std::allocator<int&>]
_GLIBCXX_ALLOC_TR_NESTED_TYPE(pointer, value_type*)
^
/usr/include/c++/4.8/bits/alloc_traits.h:100:1: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/alloc_traits.h:109:1: error: no matching function for call to 'std::allocator_traits<std::allocator<int&> >::_S_const_pointer_helper(std::allocator<int&>*)'
_GLIBCXX_ALLOC_TR_NESTED_TYPE(const_pointer,
^
/usr/include/c++/4.8/bits/alloc_traits.h:109:1: note: candidate is:
/usr/include/c++/4.8/bits/alloc_traits.h:109:1: note: template<class _Tp> static typename _Tp::const_pointer std::allocator_traits<_Alloc>::_S_const_pointer_helper(_Tp*) [with _Tp = _Tp; _Alloc = std::allocator<int&>]
_GLIBCXX_ALLOC_TR_NESTED_TYPE(const_pointer,
^
/usr/include/c++/4.8/bits/alloc_traits.h:109:1: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/alloc_traits.h:120:1: error: no matching function for call to 'std::allocator_traits<std::allocator<int&> >::_S_void_pointer_helper(std::allocator<int&>*)'
_GLIBCXX_ALLOC_TR_NESTED_TYPE(void_pointer,
^
/usr/include/c++/4.8/bits/alloc_traits.h:120:1: note: candidate is:
/usr/include/c++/4.8/bits/alloc_traits.h:120:1: note: template<class _Tp> static typename _Tp::void_pointer std::allocator_traits<_Alloc>::_S_void_pointer_helper(_Tp*) [with _Tp = _Tp; _Alloc = std::allocator<int&>]
_GLIBCXX_ALLOC_TR_NESTED_TYPE(void_pointer,
^
/usr/include/c++/4.8/bits/alloc_traits.h:120:1: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/alloc_traits.h: In substitution of 'template<class _Tp> static typename _Tp::void_pointer std::allocator_traits<_Alloc>::_S_void_pointer_helper(_Tp*) [with _Tp = _Tp; _Alloc = std::allocator<int&>] [with _Tp = std::allocator<int&>]':
/usr/include/c++/4.8/bits/alloc_traits.h:120:1: required from 'struct std::allocator_traits<std::allocator<int&> >'
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: required from 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/bits/alloc_traits.h:120:1: error: no type named 'void_pointer' in 'class std::allocator<int&>'
/usr/include/c++/4.8/bits/alloc_traits.h: In instantiation of 'struct std::allocator_traits<std::allocator<int&> >':
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: required from 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/bits/alloc_traits.h:131:1: error: no matching function for call to 'std::allocator_traits<std::allocator<int&> >::_S_const_void_pointer_helper(std::allocator<int&>*)'
_GLIBCXX_ALLOC_TR_NESTED_TYPE(const_void_pointer,
^
/usr/include/c++/4.8/bits/alloc_traits.h:131:1: note: candidate is:
/usr/include/c++/4.8/bits/alloc_traits.h:131:1: note: template<class _Tp> static typename _Tp::const_void_pointer std::allocator_traits<_Alloc>::_S_const_void_pointer_helper(_Tp*) [with _Tp = _Tp; _Alloc = std::allocator<int&>]
_GLIBCXX_ALLOC_TR_NESTED_TYPE(const_void_pointer,
^
/usr/include/c++/4.8/bits/alloc_traits.h:131:1: note: template argument deduction/substitution failed:
/usr/include/c++/4.8/bits/alloc_traits.h: In substitution of 'template<class _Tp> static typename _Tp::const_void_pointer std::allocator_traits<_Alloc>::_S_const_void_pointer_helper(_Tp*) [with _Tp = _Tp; _Alloc = std::allocator<int&>] [with _Tp = std::allocator<int&>]':
/usr/include/c++/4.8/bits/alloc_traits.h:131:1: required from 'struct std::allocator_traits<std::allocator<int&> >'
/usr/include/c++/4.8/ext/alloc_traits.h:121:10: required from 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/bits/alloc_traits.h:131:1: error: no type named 'const_void_pointer' in 'class std::allocator<int&>'
In file included from /usr/include/c++/4.8/bits/stl_construct.h:61:0,
from /usr/include/c++/4.8/bits/stl_tempbuf.h:60,
from /usr/include/c++/4.8/bits/stl_algo.h:62,
from /usr/include/c++/4.8/algorithm:62,
from /home/balajeerc/Projects/algorithms/src/main/cpp/lib/algorithms/sorting/merge_sorter.h:8,
from /home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:1:
/usr/include/c++/4.8/ext/alloc_traits.h: In instantiation of 'struct __gnu_cxx::__alloc_traits<std::allocator<int&> >':
/usr/include/c++/4.8/bits/stl_vector.h:75:28: required from 'struct std::_Vector_base<int&, std::allocator<int&> >'
/usr/include/c++/4.8/bits/stl_vector.h:210:11: required from 'class std::vector<int&, std::allocator<int&> >'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/ext/alloc_traits.h:137:23: error: no members matching '__gnu_cxx::__alloc_traits<std::allocator<int&> >::_Base_type {aka std::allocator_traits<std::allocator<int&> >}::allocate' in '__gnu_cxx::__alloc_traits<std::allocator<int&> >::_Base_type {aka struct std::allocator_traits<std::allocator<int&> >}'
using _Base_type::allocate;
^
/usr/include/c++/4.8/ext/alloc_traits.h:138:23: error: no members matching '__gnu_cxx::__alloc_traits<std::allocator<int&> >::_Base_type {aka std::allocator_traits<std::allocator<int&> >}::deallocate' in '__gnu_cxx::__alloc_traits<std::allocator<int&> >::_Base_type {aka struct std::allocator_traits<std::allocator<int&> >}'
using _Base_type::deallocate;
^
In file included from /usr/include/c++/4.8/vector:64:0,
from /usr/include/c++/4.8/bits/random.h:34,
from /usr/include/c++/4.8/random:50,
from /usr/include/c++/4.8/bits/stl_algo.h:65,
from /usr/include/c++/4.8/algorithm:62,
from /home/balajeerc/Projects/algorithms/src/main/cpp/lib/algorithms/sorting/merge_sorter.h:8,
from /home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:1:
/usr/include/c++/4.8/bits/stl_vector.h: In instantiation of 'class std::vector<int&, std::allocator<int&> >':
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:20:35: required from 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]'
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/usr/include/c++/4.8/bits/stl_vector.h:237:20: error: no members matching 'std::vector<int&, std::allocator<int&> >::_Base {aka std::_Vector_base<int&, std::allocator<int&> >}::_M_allocate' in 'std::vector<int&, std::allocator<int&> >::_Base {aka struct std::_Vector_base<int&, std::allocator<int&> >}'
using _Base::_M_allocate;
^
/usr/include/c++/4.8/bits/stl_vector.h:238:20: error: no members matching 'std::vector<int&, std::allocator<int&> >::_Base {aka std::_Vector_base<int&, std::allocator<int&> >}::_M_deallocate' in 'std::vector<int&, std::allocator<int&> >::_Base {aka struct std::_Vector_base<int&, std::allocator<int&> >}'
using _Base::_M_deallocate;
^
/usr/include/c++/4.8/bits/stl_vector.h:878:7: error: forming pointer to reference type 'int&'
data() _GLIBCXX_NOEXCEPT
^
/usr/include/c++/4.8/bits/stl_vector.h:886:7: error: forming pointer to reference type 'int&'
data() const _GLIBCXX_NOEXCEPT
^
/usr/include/c++/4.8/bits/stl_vector.h:919:7: error: 'void std::vector<_Tp, _Alloc>::push_back(std::vector<_Tp, _Alloc>::value_type&&) [with _Tp = int&; _Alloc = std::allocator<int&>; std::vector<_Tp, _Alloc>::value_type = int&]' cannot be overloaded
push_back(value_type&& __x)
^
/usr/include/c++/4.8/bits/stl_vector.h:901:7: error: with 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = int&; _Alloc = std::allocator<int&>; std::vector<_Tp, _Alloc>::value_type = int&]'
push_back(const value_type& __x)
^
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc: In instantiation of 'void sort_array(RandomAccessIterator, RandomAccessIterator) [with RandomAccessIterator = __gnu_cxx::__normal_iterator<int*, std::vector<int> >]':
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:34:56: required from here
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:22:46: error: 'merge_sort_array' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
merge_sort_array(first, last, merge_array);
^
/home/balajeerc/Projects/algorithms/src/test/cpp/lib/algorithms/sorting/test_inversion_count.cc:26:6: note: 'template<class RandomAccessIterator> void merge_sort_array(RandomAccessIterator, RandomAccessIterator, std::vector<decltype (* first)>&)' declared here, later in the translation unit
void merge_sort_array (RandomAccessIterator first, RandomAccessIterator last, std::vector<decltype(*first)>& merge_array){
^
make[2]: *** [src/test/cpp/lib/algorithms/sorting/CMakeFiles/test_inversion_count.dir/test_inversion_count.cc.o] Error 1
make[1]: *** [src/test/cpp/lib/algorithms/sorting/CMakeFiles/test_inversion_count.dir/all] Error 2
make: *** [all] Error 2
What am I doing wrong?

Answer 1

Your problem is the misuse of decltype:
decltype( expression )
yields T& for lvalue expressions of type T, and you cannot have an std::vector<int&>. Thus the error.
To fix that, use typename std::iterator_traits<RandomAccessIterator>::value_type as template parameter instead.