Question

I know it is supposed to be easy, but I just fail to get rid of the compilation errors. Here is my code:

template<class C>
struct basic_field_type_map : public hash_map<basic_string<C>, basic_string<C>>
{
};

typedef basic_field_type_map<char> field_type_map;
typedef basic_field_type_map<wchar_t> wfield_type_map;

wfield_type_map::value_type to_wfield_type_map_value(field_type_map::const_reference src)
{
  return wfield_type_map::value_type(to_wstring(src.first), to_wstring(src.second));
}

wfield_type_map to_wfield_type_map(const field_type_map& m)
{
  wfield_type_map res;
  transform(m.begin(), m.end(), res.begin(), to_wfield_type_map_value);
  return res;
}

Where to_wstring has the following signature:

wstring to_wstring(const string& s)

Compiling the code gives me the following errors:

1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(260): error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::basic_string<_Elem,_Traits,_Ax>' (or there is no acceptable conversion)
1>          with
1>          [
1>              _Elem=wchar_t,
1>              _Traits=std::char_traits<wchar_t>,
1>              _Ax=std::allocator<wchar_t>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xstring(707): could be 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(std::basic_string<_Elem,_Traits,_Ax> &&)'
1>          with
1>          [
1>              _Elem=wchar_t,
1>              _Traits=std::char_traits<wchar_t>,
1>              _Ax=std::allocator<wchar_t>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xstring(761): or       'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const std::basic_string<_Elem,_Traits,_Ax> &)'
1>          with
1>          [
1>              _Elem=wchar_t,
1>              _Traits=std::char_traits<wchar_t>,
1>              _Ax=std::allocator<wchar_t>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xstring(766): or       'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const _Elem *)'
1>          with
1>          [
1>              _Elem=wchar_t,
1>              _Traits=std::char_traits<wchar_t>,
1>              _Ax=std::allocator<wchar_t>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\xstring(771): or       'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(_Elem)'
1>          with
1>          [
1>              _Elem=wchar_t,
1>              _Traits=std::char_traits<wchar_t>,
1>              _Ax=std::allocator<wchar_t>
1>          ]
1>          while trying to match the argument list '(const std::basic_string<_Elem,_Traits,_Ax>, const std::basic_string<_Elem,_Traits,_Ax>)'
1>          with
1>          [
1>              _Elem=wchar_t,
1>              _Traits=std::char_traits<wchar_t>,
1>              _Ax=std::allocator<wchar_t>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(259) : while compiling class template member function 'std::pair<_Ty1,_Ty2> &std::pair<_Ty1,_Ty2>::operator =(std::pair<_Ty1,_Ty2> &&)'
1>          with
1>          [
1>              _Ty1=const std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>>,
1>              _Ty2=std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>>
1>          ]
1>          z:\dev\internal\vedtool\defs.h(34) : see reference to class template instantiation 'std::pair<_Ty1,_Ty2>' being compiled
1>          with
1>          [
1>              _Ty1=const std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>>,
1>              _Ty2=std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>>
1>          ]
Build has been canceled.

I am using VS2010.

What am I doing wrong?

Thanks.

Answer 1

The keytype in a map is const, you can’t assign to it, which is what std::transform does; the loop looks something like this:

for(; first != last; ++first, ++dest)
  *dest = func(*first);  // assignment of key-value pair here

Note that there is an even bigger problem: You res map doesn’t even have the space to hold all the stuff from the field_type_map! You’re invoking undefined behaviour here by just writing past the end.

Your best bet is to manually insert each new pair with for_each:

std::for_each(m.begin(), m.end(),
    [&](field_type_map::value_type const& p){
      res.insert(to_wfield_type_map_value(p));
    });