#include <string>
  size_type find_first_of( const string &str, size_type index = 0 );
  size_type find_first_of( const char* str, size_type index = 0 );
  size_type find_first_of( const char* str, size_type index, size_type num );
  size_type find_first_of( char ch, size_type index = 0 );

The find_first_of() function either:

• returns the index of the first character within the current string that matches any character in str, beginning the search at index, string::npos if nothing is found,
• returns the index of the first character within the current string that matches any character in str, beginning the search at index and searching at most num characters, string::npos if nothing is found,
• or returns the index of the first occurrence of ch in the current string, starting the search at index, string::npos if nothing is found.