Language: C++

#include <bits/stdc++.h>
using namespace std;

int main() {
  int H, W;
  cin >> H >> W;
  
  vector<char> a((H+2)*(W+2), "#");
  for (int i=0; i<(H+2)*(W+2); i++) {
    if ( (i !< W+2) || (i !> (H+2)*(W+2)-(W+2)-1 ) ) {
      if ( (i%(W-1)==0) || (i%*W==0) ){
        cin >> a.at(i);
      }
    }
  }
  
  for (int i=0; i<(H+2)*(W+2); i++) {
    cout << a.at(i);
    if(i%(W-1)==0) cout << endl;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:34: error: no matching function for call to 'std::vector<char>::vector(int, const char [2])'
    8 |   vector<char> a((H+2)*(W+2), "#");
      |                                  ^
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_vector.h:569:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const value_type&, const allocator_type&) [with _Tp = char; _Alloc = std::allocator<char>; size_type = long unsigned int; value_type = char; allocator_type = std::allocator<char>]' (near match)
  569 |       vector(size_type __n, const value_type& __value,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:569:7: note:   conversion of argument 2 would be ill-formed:
a.cc:8:31: error: invalid conversion from 'const char*' to 'std::vector<char>::value_type' {aka 'char'} [-fpermissive]
    8 |   vector<char> a((H+2)*(W+2), "#");
      |                               ^~~
      |                               |
      |                               const char*
/usr/include/c++/14/bits/stl_vector.h:707:9: note: candidate: 'template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with <template-parameter-2-2> = _InputIterator; _Tp = char; _Alloc = std::allocator<char>]'
  707 |         vector(_InputIterator __first, _InputIterator __last,
      |         ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:707:9: note:   template argument deduction/substitution failed:
a.cc:8:34: note:   deduced conflicting types for parameter '_InputIterator' ('int' and 'const char*')
    8 |   vector<char> a((H+2)*(W+2), "#");
      |                                  ^
/usr/include/c++/14/bits/stl_vector.h:678:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = char; _Alloc = std::allocator<char>; allocator_type = std::allocator<char>]'
  678 |       vector(initializer_list<value_type> __l,
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:678:43: note:   no known conversion for argument 1 from 'int' to 'std::initializer_list<char>'
  678 |       vector(initializer_list<value_type> __l,
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:659:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, std::__type_identity_t<_Alloc>&) [with _Tp = char; _Alloc = std::allocator<char>; std::__type_identity_t<_Alloc> = std::allocator<char>]'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:659:23: note:   no known conversion for argument 1 from 'int' to 'std::vector<char>&&'
  659 |       vector(vector&& __rv, const __type_identity_t<allocator_type>& __m)
      |              ~~~~~~~~~^~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::false_type) [with _Tp = char; _Alloc = std::allocator<char>; allocator_type = std::allocator<char>; std::false_type = std::false_type]'
  640 |       vector(vector&& __rv, const allocator_type& __m, false_type)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:640:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:635:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&, const allocator_type&, std::true_type) [with _Tp = char; _Alloc = std::allocator<char>; allocator_type = std::allocator<char>; std::true_type = std::true_type]'
  635 |       vector(vector&& __rv, const allocator_type& __m, true_type) noexcept
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:635:7: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_vector.h:624:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&, std::__type_identity_t<_Alloc>&) [with _Tp = char; _Alloc = std::allocator<char>; std::__type_identity_t<_Alloc> = std::allocator<char>]'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:624:28: note:   no known conversion for argument 1 from 'int' to 'const std::vector<char>&'
  624 |       vector(const vector& __x, const __type_identity_t<allocator_type>& __a)
      |              ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(std::vector<_Tp, _Alloc>&&) [with _Tp = char; _Alloc = std::allocator<char>]'
  620 |       vector(vector&&) noexcept = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:620:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:601:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = char; _Alloc = std::allocator<char>]'
  601 |       vector(const vector& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:601:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:556:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(size_type, const allocator_type&) [with _Tp = char; _Alloc = std::allocator<char>; size_type = long unsigned int; allocator_type = std::allocator<char>]'
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:556:51: note:   no known conversion for argument 2 from 'const char [2]' to 'const std::vector<char>::allocator_type&' {aka 'const std::allocator<char>&'}
  556 |       vector(size_type __n, const allocator_type& __a = allocator_type())
      |                             ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector(const allocator_type&) [with _Tp = char; _Alloc = std::allocator<char>; allocator_type = std::allocator<char>]'
  542 |       vector(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:542:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_vector.h:531:7: note: candidate: 'std::vector<_Tp, _Alloc>::vector() [with _Tp = char; _Alloc = std::allocator<char>]'
  531 |       vector() = default;
      |       ^~~~~~
/usr/include/c++/14/bits/stl_vector.h:531:7: note:   candidate expects 0 arguments, 2 provided
a.cc:10:12: error: expected ')' before '!' token
   10 |     if ( (i !< W+2) || (i !> (H+2)*(W+2)-(W+2)-1 ) ) {
      |          ~ ^~
      |            )
a.cc:14:6: error: expected ')' before '}' token
   14 |     }
      |      ^
      |      )
   15 |   }
      |   ~   
a.cc:10:8: note: to match this '('
   10 |     if ( (i !< W+2) || (i !> (H+2)*(W+2)-(W+2)-1 ) ) {
      |        ^
a.cc:15:3: error: expected primary-expression before '}' token
   15 |   }
      |   ^