I'll go aswell with the stringstream methods. It's form the standard libraries, and it's not that hard to implement. Have a look at the link that mike_2000_17 posted, you'll find everything you need there.
If you fancy, you can use the istringstream (which manipulates strings as input streams) or/with ostringstream (which manipulate strings as is they were output streams) but basically stringstream acts both as istringstreams and ostringstreams, but if you need only to work with input streams, or outpustreams, perhaps i/ostringstreams would suite you better.
using namespace std;
get_int>>value; // string to int as input stream.
get_string<<value; //int to string as output stream
cout<<"Int 42+5: "<<value+5<<endl;
cout<<"String 42+\"yh\": "<<value_s+"yh";