[FIXED] Why does std::begin behave differently when called in a nested function

Issue

I have some simple code

#include<iterator>

int main() {
    int y[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
    auto a = std::begin(y);
    std::cout << *a << std::endl;
    return 0;
}

Which prints out 1 as expected.

However if I do this :

void checkNested(int val [10]) {
    auto a = std::begin(val);
    std::cout << *a << std::endl;

}

int main() {
    int y[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
    checkNested(y);
    return 0;
}

I get compilation failures from both clang++ and g++. From clang++ specifically I get:

    auto a = std::begin(input);
             ^~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/initializer_list:89:5: note: candidate template ignored: could not match 'initializer_list<type-parameter-0-0>' against 'int *'
    begin(initializer_list<_Tp> __ils) noexcept
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/range_access.h:48:5: note: candidate template ignored: substitution failure [with _Container = int *]: member reference base type 'int *' is not a structure or union
    begin(_Container& __cont) -> decltype(__cont.begin())
    ^                                           ~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/range_access.h:58:5: note: candidate template ignored: substitution failure [with _Container = int *]: member reference base type 'int *const' is not a structure or union
    begin(const _Container& __cont) -> decltype(__cont.begin())
    ^                                                 ~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/range_access.h:87:5: note: candidate template ignored: could not match '_Tp [_Nm]' against 'int *'
    begin(_Tp (&__arr)[_Nm])
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/range_access.h:104:31: note: candidate template ignored: could not match 'valarray<type-parameter-0-0>' against 'int *'

  template<typename _Tp> _Tp* begin(valarray<_Tp>&);
                              ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/range_access.h:105:37: note: candidate template ignored: could not match 'valarray<type-parameter-0-0>' against 'int *'
  template<typename _Tp> const _Tp* begin(const valarray<_Tp>&);

Just wanna know if there's something really obvious I'm missing here since I expected them to function the same. Thanks

Solution

An array can't be passed by value, so your array decays into a pointer when passed to checkNested(), and std::begin() is not defined for a pointer, hence the error.

void checkNested(int val [10]) is just syntax sugar for void checkNested(int *val).

If you pass the array by reference instead, then the code will work:

void checkNested(int (&val) [10])

Answered By - Remy Lebeau

Answer Checked By - Marie Seifert (PHPFixing Admin)