auto it = vector.begin () el tipo resultante no es convertible a const_iterator
Se requiere que los contenedores proporcionen uniterator
tipo que es convertible implícitamente a unconst_iterator
. Dado esto, estoy tratando de usarauto
para inicializar un objeto a través devector::begin()
y usar ese objeto resultante enstd::distance
donde el RHS es unconst_iterator
. Esto no esta funcionando. Aquí hay un ejemplo completo:
#include <cstdlib>
#include <vector>
#include <iterator>
#include <iostream>
typedef std::vector <char> Packet;
typedef std::vector <Packet> Packets;
template <typename Iter>
Iter next_upto (Iter begin, Iter end, size_t n)
{
Iter ret = begin;
for (; n > 0 && ret != end; ++ret, --n)
;
return ret;
}
Packets::const_iterator Process (Packets::const_iterator begin, Packets::const_iterator end)
{
Packets::const_iterator ret = begin;
while (ret != end)
++ret; // do something
return ret;
}
int main()
{
Packets test (100); // vector of 100 default-initialized packets
// process them 10 at a time
for (auto it = test.begin();
it != test.end();
it = next_upto (it, test.end(), 10))
{
auto itr = Process (it, next_upto (it, test.end(), 10));
Packets::const_iterator it2 = it;
const size_t n1 = std::distance (it2, itr);
const size_t n = std::distance (it, itr);
std::cout << "Processed " << n << " packets\n";
}
}
En g ++ 4.8.1 (y 4.8.2) compilar esto produce:
[1/2] Building CXX object CMakeFiles/hacks.dir/main.o
FAILED: /usr/bin/g++ -Wall -std=c++11 -g -MMD -MT CMakeFiles/hacks.dir/main.o -MF "CMakeFiles/hacks.dir/main.o.d" -o CMakeFiles/hacks.dir/main.o -c main.cpp
main.cpp: In function ‘int main()’:
main.cpp:39:45: error: no matching function for call to ‘distance(__gnu_cxx::__normal_iterator<std::vector<char>*, std::vector<std::vector<char> > >&, __gnu_cxx::__normal_iterator<const std::vector<char>*, std::vector<std::vector<char> > >&)’
const size_t n = std::distance (it, itr);
^
main.cpp:39:45: note: candidate is:
In file included from /usr/include/c++/4.8/bits/stl_algobase.h:66:0,
from /usr/include/c++/4.8/vector:60,
from main.cpp:2:
/usr/include/c++/4.8/bits/stl_iterator_base_funcs.h:114:5: note: template<class _InputIterator> typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator)
distance(_InputIterator __first, _InputIterator __last)
^
/usr/include/c++/4.8/bits/stl_iterator_base_funcs.h:114:5: note: template argument deduction/substitution failed:
main.cpp:39:45: note: deduced conflicting types for parameter ‘_InputIterator’ (‘__gnu_cxx::__normal_iterator<std::vector<char>*, std::vector<std::vector<char> > >’ and ‘__gnu_cxx::__normal_iterator<const std::vector<char>*, std::vector<std::vector<char> > >’)
const size_t n = std::distance (it, itr);
^
Soy consciente de que puedo solucionar esta instancia en particular llamando acbegin()
ycend()
más bien quebegin()
yend()
, pero desdebegin()
yend()
devolver un tipo que debería ser convertible aconst_iterator
, No estoy seguro de entender por qué esto es necesario.
Por queauto
deduzca un tipo en este caso que no sea convertible aconst_iterator
?