Вывод типа 'auto' при использовании цикла range-for в многомерных массивах

Question

После int ia[3][4]{} я запускаю:

for(auto row : ia) // row should be type int*
    for(int *j = std::begin(*row); j!= end(*row); ++j) // error!!
        std::cout << *j << std::endl;

В соответствии с праймером C ++ 5:

Поскольку строка не является ссылкой, когда компилятор
инициализирует строкуон преобразует каждый элемент массива (как и любой другой
объект типа массива)
в указатель на первый элемент этого массива

Так что, если строка является указателем на iaпервый элемент, то почему происходит ошибка?

Большое спасибо!

ОБНОВЛЕНИЕ:

ch339.cpp: In function ‘int main()’:
ch339.cpp:10:31: error: no matching function for call to ‘begin(int*&)’
        for (int *j = begin(row); j!= end(row); ++j )
                               ^
ch339.cpp:10:31: note: candidates are:
In file included from /usr/include/c++/4.9/bits/basic_string.h:42:0,
                 from /usr/include/c++/4.9/string:52,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ch339.cpp:1:
/usr/include/c++/4.9/initializer_list:89:5: note: template<class _Tp> constexpr const _Tp* std::begin(std::initializer_list<_Tp>)
     begin(initializer_list<_Tp> __ils) noexcept
     ^
/usr/include/c++/4.9/initializer_list:89:5: note:   template argument deduction/substitution failed:
ch339.cpp:10:31: note:   mismatched types ‘std::initializer_list<_Tp>’ and ‘int*’
        for (int *j = begin(row); j!= end(row); ++j )
                               ^
In file included from /usr/include/c++/4.9/string:51:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ch339.cpp:1:
/usr/include/c++/4.9/bits/range_access.h:87:5: note: template<class _Tp, long unsigned int _Nm> _Tp* std::begin(_Tp (&)[_Nm])
     begin(_Tp (&__arr)[_Nm])
     ^
/usr/include/c++/4.9/bits/range_access.h:87:5: note:   template argument deduction/substitution failed:
ch339.cpp:10:31: note:   mismatched types ‘_Tp [_Nm]’ and ‘int*’
        for (int *j = begin(row); j!= end(row); ++j )
                               ^
In file included from /usr/include/c++/4.9/string:51:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ch339.cpp:1:
/usr/include/c++/4.9/bits/range_access.h:58:5: note: template<class _Container> decltype (__cont.begin()) std::begin(const _Container&)
     begin(const _Container& __cont) -> decltype(__cont.begin())
     ^
/usr/include/c++/4.9/bits/range_access.h:58:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of ‘template<class _Container> decltype (__cont.begin()) std::begin(const _Container&) [with _Container = int*]’:
ch339.cpp:10:31:   required from here
/usr/include/c++/4.9/bits/range_access.h:58:5: error: request for member ‘begin’ in ‘__cont’, which is of non-class type ‘int* const’
/usr/include/c++/4.9/bits/range_access.h:48:5: note: template<class _Container> decltype (__cont.begin()) std::begin(_Container&)
     begin(_Container& __cont) -> decltype(__cont.begin())
     ^
/usr/include/c++/4.9/bits/range_access.h:48:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of ‘template<class _Container> decltype (__cont.begin()) std::begin(_Container&) [with _Container = int*]’:
ch339.cpp:10:31:   required from here
/usr/include/c++/4.9/bits/range_access.h:48:5: error: request for member ‘begin’ in ‘__cont’, which is of non-class type ‘int*’
ch339.cpp:10:45: error: no matching function for call to ‘end(int*&)’
        for (int *j = begin(row); j!= end(row); ++j )
                                             ^
ch339.cpp:10:45: note: candidates are:
In file included from /usr/include/c++/4.9/bits/basic_string.h:42:0,
                 from /usr/include/c++/4.9/string:52,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ch339.cpp:1:
/usr/include/c++/4.9/initializer_list:99:5: note: template<class _Tp> constexpr const _Tp* std::end(std::initializer_list<_Tp>)
     end(initializer_list<_Tp> __ils) noexcept
     ^
/usr/include/c++/4.9/initializer_list:99:5: note:   template argument deduction/substitution failed:
ch339.cpp:10:45: note:   mismatched types ‘std::initializer_list<_Tp>’ and ‘int*’
        for (int *j = begin(row); j!= end(row); ++j )
                                             ^
In file included from /usr/include/c++/4.9/string:51:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ch339.cpp:1:
/usr/include/c++/4.9/bits/range_access.h:97:5: note: template<class _Tp, long unsigned int _Nm> _Tp* std::end(_Tp (&)[_Nm])
     end(_Tp (&__arr)[_Nm])
     ^
/usr/include/c++/4.9/bits/range_access.h:97:5: note:   template argument deduction/substitution failed:
ch339.cpp:10:45: note:   mismatched types ‘_Tp [_Nm]’ and ‘int*’
        for (int *j = begin(row); j!= end(row); ++j )
                                             ^
In file included from /usr/include/c++/4.9/string:51:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from ch339.cpp:1:
/usr/include/c++/4.9/bits/range_access.h:78:5: note: template<class _Container> decltype (__cont.end()) std::end(const _Container&)
     end(const _Container& __cont) -> decltype(__cont.end())
     ^
/usr/include/c++/4.9/bits/range_access.h:78:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of ‘template<class _Container> decltype (__cont.end()) std::end(const _Container&) [with _Container = int*]’:
ch339.cpp:10:45:   required from here
/usr/include/c++/4.9/bits/range_access.h:78:5: error: request for member ‘end’ in ‘__cont’, which is of non-class type ‘int* const’
/usr/include/c++/4.9/bits/range_access.h:68:5: note: template<class _Container> decltype (__cont.end()) std::end(_Container&)
     end(_Container& __cont) -> decltype(__cont.end())
     ^
/usr/include/c++/4.9/bits/range_access.h:68:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of ‘template<class _Container> decltype (__cont.end()) std::end(_Container&) [with _Container = int*]’:
ch339.cpp:10:45:   required from here
/usr/include/c++/4.9/bits/range_access.h:68:5: error: request for member ‘end’ in ‘__cont’, which is of non-class type ‘int*’

shell returned 1

Press ENTER or type command to continue

Answer 1

Тип row определяется как int*.Это означает, что, как и любой другой int*, компилятор не знает, насколько велик массив, на который он указывает, и даже не является указателем на первый элемент массива вообще.Вся эта информация теряется, когда массив превращается в указатель на его первый элемент.

Если вместо этого вы используете что-то вроде

for (auto& row : ia) // <-- NOTE: row is now a reference
    for (int* j = std::begin(row); j != std::end(row); ++j)
        std::cout << *j << '\n';

, тогда тип row будет выведен вint (&)[4]: ссылка на массив 4 int с.Информация о длине сохраняется, поэтому std::begin и std::end имеют необходимую информацию.

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PS: Как примечание: диапазон для работы с использованием std::begin и std::end внутриТаким образом, вышеприведенное может быть более кратко записано как

for (auto& row : ia)
    for (auto j : row)
        std::cout << j << '\n';