How to convert a large integer from a string to a uint64_t?

Question

I have a large integer stored as a string. I need to convert it into an integer ( uint64_t ). I tried stoi() , but it is crashing after throwing an instance of std::out_of_range .

The string has maximum of 64-bit integer value, max value of 18,446,744,073,709,551,615 (maximum for an unsigned long int).

How do I do this, other than (of course) manually?

Answer 1

As long as the string contains a number that is less than std::numeric_limits<uint64_t>::max() , then std::stoull() will do what you're expecting.

(The std::stoull() function is new in C++11.)

Answer 2

You can use stringstream to do this. This is supported in C++98 (the std::stoull() function was added in C++11).

#include <stdint.h>
#include <iostream>
#include <sstream>
using namespace std;

uint64_t string_to_uint64(string str) {
  stringstream stream(str);
  uint64_t result;
  stream >> result;
  return result;
}

int main() {
  string str = "1234567891234567";
  uint64_t val = string_to_uint64(str);
  cout << val;
  return 0;
}

Answer 3

One approach could be to use stringstreams:

unsigned long long val;
std::istringstream stream(inputstring);
stream >> val;

According to this reference there's at least an overload for operator>> for unsigned long long , which could be the same as uint64_t .

Otherwise you'll probably have to do it manually, eg split the string into handleable pieces, and join the results by multiplication with the right base.

---

To be sure I'd go with this for now:

unit64_t to_uint64(std::string const & in) {
  static_assert(sizeof(unsigned long long) == sizeof(uint64_t), "ull not large enough");
  unsigned long long val = 0;
  std::istringstream stream(in);
  stream >> val;
  return val;
}