c++ - num_get facet and stringstream conversion to boolean - fails with initialised boolean? -

i have inherited template convert string numerical value, , want apply convert boolean. not experienced stringstream , locale classes. seem getting odd behaviour, , wondering if please explain me?

template<typename t> t convertfromstring( const string& str ) const {  std::stringstream sstream( str );   t num = 0;  sstream >> num;  return num;   }   

this works fine until try boolean conversion

string str1("1"); int val1 = convertfromstring<int>(str1); // ok string str2("true"); bool val2 = convertfromstring<bool>(str2); // val2 _false_ 

i spent time tracking down problem. have confirmed locale's truename() returns "true".

the problem seems initialisation of variable num. can change template , works:

template<typename t> t convertfromstring( const string& str ) const {  std::stringstream sstream( str );   t num;  // <----------------------- changed here sstream >> num;  return num;   }   string str2("true"); bool val2 = convertfromstring<bool>(str2); // val2 _true_ 

why work? accept initialising bool '0' wrong, why cause sstream>>numconversion fail?

initialising bool 0 reliably set false, , has no effect on stream extraction.

what causing problem streams default recognize values 0 , 1 when dealing booleans. have them recognize names true , false, need tell explicitly stream boolalpha manipulator.

the best way solve problems specialize template bool:

template<> bool convertfromstring<bool>( const string& str ) const {   std::stringstream sstream( str );     bool val = false;   sstream >> val;   if( sstream.fail() )   {     sstream.clear();      sstream >> boolalpha >> val;    }       return val;   }   

note change did not make code work. appeared single testcase used. change, function failed read stream , returned uninitialised value. non-zero value interpreted true, function appears work, try extract "false", see fail (the function still returns true).

edit: adapted code handle both numeric , alpha bools.