Question

I do not understand why the following code is returning *end=='\0\'?

bool isValue(const string &token,int &output_value) //czy string jest wartoscia
{
  char *end = 0;
  errno=0;
  output_value = strtol(token.c_str(),&end,10); //converts string to int
  if (errno!=0) return false;
  return *end=='\0';
}

EDIT: And stupid question but I don’t know why there is

bool isValue(const string &token,int &output_value)

instead of

bool isValue(string &token,int &output_value)

and

bool isValue(string token,int output_value)

Answer 1

The errno part is probably legacy code. It could have been used at some point, but the code that used it could have been removed, but errno forgotten. You’re right, it does nothing and will probably be optimized out. <— thought errno was a local variable (stealth edit?)

end is

a pointer to the first character following the integer representation in str is stored in the object pointed by endptr.

so basically the condition checks whether there were any other characters in the string after the number.