In C++,

Convert int to string:

#include <sstream>
string int2str(int n) {
    stringstream o;
    o << n;
    return o.str();
}

Convert string to int:

#include <sstream>
int str2int(string s) {
    int n;
    istringstream iss(s);
    iss >> n;
    return n;
}

Conversion of string and double etc. is similar.

To convert from int or other types to string, you can also use function to_string(). 

Note: to remember which stringstream type to use, you can think this way:
- if return type is int, then use istringstream (starting with "i" or "int")
- if return type is string, then use string stream (string with "s" or "string")

See: http://www.cplusplus.com/reference/string/to_string/. 
To use the to_string() function, include header: #include <string>. 
In C++11, also in GCC4.5, compile with -std=c++0x flag.

------------
Another way of converting string to int:

int strToInt(string s) {
    int d;
    sscanf(s.c_str(), "%d", &d);
    return d;
}