Cannot convert a character/string to int - c++

Question

When I run my code I get this error at compile time:
# g++ -std=c++0x sixteen.cpp -O3 -Wall -g3 -o sixteen
sixteen.cpp: In function ‘int main()’:
sixteen.cpp:10: error: call of overloaded ‘stoi(char&)’ is ambiguous
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/basic_string.h:2565: note: candidates are: int std::stoi(const std::string&, size_t*, int) <near match>
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/basic_string.h:2626: note: int std::stoi(const std::wstring&, size_t*, int) <near match>
I looked up that error and followed the instructions that other questions on here have done, however I still get that error after removing using namespace std;. Why is this still happening and what can I do to get rid of it?
Code:
#include <iostream>
#include <string>
int main() {
std::string test = "Hello, world!";
std::string one = "123";
std::cout << "The 3rd index of the string is: " << test[3] << std::endl;
int num = std::stoi(one[2]);
printf( "The 3rd number is: %d\n", num );
return 0;
}

Answer 1

std::stoi takes a std::string as its argument, but one[2] is a char.
The easiest way to fix this is to use the fact that the digit characters are guaranteed to have contiguous values, so you can do:
int num = one[2] - '0';
Alternatively, you can extract the digit as a substring:
int num = std::stoi(one.substr(2,1));
And another alternative, you can construct a std::string using the constructor that takes a char and the number of times that char should appear:
int num = std::stoi(std::string(1, one[2]));