Convert string to unsigned long long integer
Parses the C-string str interpreting its content as an integral number of the specified base, which is returned as a value of type unsigned long long int. If endptr is not a null pointer, the function also sets the value of endptr to point to the first character after the number.
This function operates like strtol to interpret the string, but produces numbers of type unsigned long long int (see strtol for details on the interpretation process).
strtoull
Parameters
str
C-string beginning with the representation of an integral number.
endptr
Reference to an object of type char*, whose value is set by the function to the next character in str after the numerical value.
This parameter can also be a null pointer, in which case it is not used.
base
Numerical base (radix) that determines the valid characters and their interpretation.
If this is 0, the base used is determined by the format in the sequence (see strtol for details).
Return Value
On success, the function returns the converted integral number as an unsigned long long int value.
If no valid conversion could be performed, a zero value is returned (0ULL).
If the value read is out of the range of representable values by an unsigned long long int, the function returns ULLONG_MAX (defined in <climits>), and errno is set to ERANGE.
Fonte:
http://www.cplusplus.com/reference/cstdlib/strtoull/