Turning a string into an integer?

Question

I'm making a function that inputs a string class number and translates it into integers.

for example. I punch in 123 I'll get back 123 as an integer, or I punch in 1D2F I...guess I get it back? But I figured I'll turn any base number back to decimal. (But how would I make this string into a decimal if I'm not completely sure you can do math effectively with strings?

So far for my stringToInt function I have.

int StringToInt (string inputString){
   int i;
   int newIntegerLine;

      for (i=0; i<inputString.length();i++){
         //subtracts the ascii number of 0 from
         //whatever character you input
         newIntegerLine= (inputString.at(i)- '0');
      }

   return newIntegerLine;
}

I figured I can use the ascii numbers to get characters to integers. But when I run it whatever I put it is returned as 0. And I really am not sure how to approach the base number problem (What to do with A-F, perhaps if statements? Is there a more elegant way of doing this?). Can I call my base function within my StringToInt function? Or is there already a function I can use to accomplish this? Am I just complicating matters?

my base function (Which seems to work I guess? There seems to be a slight issue with binary numbers when I punch in 100 and say its in base 2 I get 24 back as it's decimal equivalent. Otherwise it works perfectly.)

int baseToDecimal (int numInput, int base){
   int i, modSum;
   modSum=numInput%10;
   for(i=base;(numInput/=10)!=0;i*=base)
      modSum+=numInput*i;
   return modSum;
   }

Answer 1

The old C way (atoi):

std::string foo = "1337";
int bar = atoi(foo.c_str());

Using std::istringstream:

std::string foo = "1337";
int bar;
std::istringstream(foo) >> bar;

C++11's std::stoi:

std::string foo = "1337";
int bar = std::stoi(foo);

which under the hood uses std::strtol:

std::string foo = "1337";
int bar = std::strtol(foo.str(), nullptr, 10);

And to add an example for @polkadotcadaver's mention of boost::lexical_cast:

std::string foo = "1337";
int bar = boost::lexical_cast<int>(foo);

Don't forget to add appropriate error handling!