Help with switch and getopt

I am trying to read the arguments in the main with this code, what I need is that, when the arguments are -v then a = 10 keep this and when they are -g pass flag a true , but when I run it with -g send me "file: unknown option - g"

string val;
string name;
bool flag=false;
int c;
while((c=getopt(argc,argv,"vg:")) != -1){
  string a;
  bool b;
  int va;
  switch(c){
  case 'v':
    b=false;
    a=optarg;      
    for (int i=0;i<a.length();i++){
       if(a[i]!='='&& b==false)
         {
          name+=a[i];
         }
      if(a[i]!='='&& b==true)
        {
         val+=a[i];
        }
     if(a[i]=='=')
        {
         b=true;
        }
     }

    va=atoi(val.c_str());
    calc->addVal(name,va);
    name.clear();
    val.clear();
    a.clear();
    break;

 case 'g':

   flag=true;
   cout<<flag<<endl;
   break;


  }
}