Changing integer to binary string of digits

There are actually standard one-liners for these.

#include <bitset>

std::string s = std::bitset< 64 >( 12345 ).to_string(); // string conversion

std::cout << std::bitset< 64 >( 54321 ) << ' '; // direct output

std::bitset< 64 > input;
std::cin >> input;
unsigned long ul = input.to_ulong();

See this run as a demo.