Convert a string into a double, and a int. How to? - c++

Question

I'm having trouble converting a string into a double.
My string has been declared using the "string" function, so my string is:
string marks = "";
Now to convert it to a double I found somewhere on the internet to use word.c_str(), and so I did. I called it and used it like this:
doubleMARK = strtod( marks.c_str() );
This is similar to the example I found on the web:
n1=strtod( t1.c_str() );
Apparently, that's how it's done. But of course, it doesn't work. I need another parameter. A pointer I believe? But I'm lost at this point as to what I'm suppose to do. Does it need a place to store the value or something? or what?
I also need to convert this string into a integer which I have not begun researching as to how to do, but once I find out and if I have errors, I will edit this out and post them here.

Answer 1

Was there a reason you're not using std::stod and std::stoi? They are at least 9 levels more powerful than flimsy strtod.
Example
#include <iostream>
#include <string>
int main() {
using namespace std;
string s = "-1";
double d = stod(s);
int i = stoi(s);
cout << s << " " << d << " " << i << endl;
}
Output
-1 -1 -1
If you must use strtod, then just pass NULL as the second parameter. According to cplusplus.com:
If [the second parameter] is not a null pointer, the function also sets the value pointed by endptr to point to the first character after the number.
And it's not required to be non-NULL.

Answer 2

Back in the Bad Old Dark Days of C, I'd do something ugly and unsafe like this:
char sfloat[] = "1.0";
float x;
sscanf (sfloat, "%lf", &x);
In C++, you might instead do something like this:
// REFERENCE: http://www.codeguru.com/forum/showthread.php?t=231054
include <string>
#include <sstream>
#include <iostream>
template <class T>
bool from_string(T& t,
const std::string& s,
std::ios_base& (*f)(std::ios_base&))
{
std::istringstream iss(s);
return !(iss >> f >> t).fail();
}
int main()
{
int i;
float f;
// the third parameter of from_string() should be
// one of std::hex, std::dec or std::oct
if(from_string<int>(i, std::string("ff"), std::hex))
{
std::cout << i << std::endl;
}
else
{
std::cout << "from_string failed" << std::endl;
}
if(from_string<float>(f, std::string("123.456"), std::dec))
{
std::cout << f << std::endl;
}
else
{
std::cout << "from_string failed" << std::endl;
}
return 0;
}
Personally, though, I'd recommend this:
http://bytes.com/topic/c/answers/137731-convert-string-float
There are two ways. C gives you strtod which converts between a char
array and double:
// C-ish:
input2 = strtod(input.c_str(), NULL);
The C++ streams provide nice conversions to and from a variety of
types. The way to use strings with streams is to use a stringstream:
// C++ streams:
double input2;
istringstream in(input);
input >> input2;

Answer 3

We can define a stringTo() function,
#include <string>
#include <sstream>
template <typename T>
T stringTo(const std::string& s) {
T x;
std::istringstream in(s);
in >> x;
return x;
}
Then, use it like
std::cout << stringTo<double>("-3.1e3") << " " << stringTo<int>("4");