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#include <iostream>
#include <string>
using std::string;
struct BB
{
 // generic cast
 template<typename T>
 operator T() const
 {   
   std::cout<<"Generic cast\n";
   return 0;
 }
 
 // string cast
 operator string() const
 { 
   std::cout<<"string cast\n";
   return string("hello");
 }
};
int main()
{
  BB b;
  string s ;
  s= b;
}

prog.cpp: In function 'int main()':
prog.cpp:25:4: error: ambiguous overload for 'operator=' (operand types are 'std::string {aka std::basic_string<char>}' and 'BB')
   s= b;
    ^
In file included from /usr/include/c++/5/string:52:0,
                 from /usr/include/c++/5/bits/locale_classes.h:40,
                 from /usr/include/c++/5/bits/ios_base.h:41,
                 from /usr/include/c++/5/ios:42,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/5/bits/basic_string.h:2950:7: note: candidate: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       operator=(const basic_string& __str) 
       ^
/usr/include/c++/5/bits/basic_string.h:2958:7: note: candidate: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       operator=(const _CharT* __s) 
       ^
/usr/include/c++/5/bits/basic_string.h:2969:7: note: candidate: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       operator=(_CharT __c) 
       ^
/usr/include/c++/5/bits/basic_string.h:2985:7: note: candidate: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(std::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       operator=(basic_string&& __str)
       ^
/usr/include/c++/5/bits/basic_string.h:2997:7: note: candidate: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(std::initializer_list<_Tp>) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       operator=(initializer_list<_CharT> __l)
       ^

Standard output is empty