c++ - Why does the following validation code get stuck on the cin.ignore when I enter 1.w and go into an infinite loop when I enter w.1? -

why following validation code stuck on cin.ignore when enter 1.w , go infinite loop when enter w.1?
trying create code validates numerical input. have created code suggestions given on other posts, i'm still having problems.

//the code validation code used check if input numerical (float or integer).  #include <iostream> #include <string> #include <limits> // std::numeric_limits using namespace std; int main () {     string line;     float amount=0;     bool flag=true;  //while loop check inputs while (flag){ //check valid numerical input     cout << "enter amount:";     getline(cin>>amount,line); //use string find_first_not_of function test numerical input     unsigned test = line.find_first_not_of('0123456789-.');      if (test==std::string::npos){ //if input stream contains valid inputs         cout << "wow!" << endl;         cout << "you entered " << line << endl;         cout << "amount = " << amount << endl;     }      else{ //if input stream invalid         cin.clear();         // ignore end of line         cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');     }   }      return 0; } 

first, '0123456789-.' should "0123456789-." (note double quotes). former multibyte character literal.

when enter 1.w:

• 1 gets extracted cin>>amount.
• .w extracted getline
• the stream empty, ignore waits inputs

when enter w.1:

• cin>>amount fails, failbit gets set
• getline can't extract when stream bad, line stays empty
• test equals npos, never enter else block clear stream
• repeat on again