There are two common methods to convert strings to numbers:

Using stringstream class or sscanf() 
stringstream() : This is an easy way to convert strings of digits into ints, floats or doubles. Following is a sample program using a stringstream to convert string to int.

CPP

// A program to demonstrate the use of stringstream
#include 
#include 
using namespace std;

int main()
{
	string s = "12345";

	// object from the class stringstream
	stringstream geek(s);

	// The object has the value 12345 and stream
	// it to the integer x
	int x = 0;
	geek >> x;

	// Now the variable x holds the value 12345
	cout << "Value of x : " << x;

	return 0;
}

Output:

Value of x : 12345
// A stringstream is similar to input/output
// file stream. We need to declare a stringstream
// just like an fstream, for example: 
stringstream ss;

// and, like an fstream or cout, 
// we can write to it:
ss << myString; or 
ss << myCstring; or
ss << myInt;, or float, or double, etc.

// and we can read from it:
ss >> myChar; or
ss >> myCstring; or
ss >> myInt;  

To summarize, stringstream is a convenient way to manipulate strings.
sscanf() is a C style function similar to scanf(). It reads input from a string rather that standard input. 

CPP

#include
int main()
{
	const char *str = "12345";
	int x;
	sscanf(str, "%d", &x);
	printf("\nThe value of x : %d", x);
	return 0;
}

Output:

Value of x : 12345

Similarly we can read float and double using %f and %lf respectively.

String conversion using stoi() or atoi() 
stoi() : The stoi() function takes a string as an argument and returns its value. Following is a simple implementation:

// C++ program to demonstrate working of stoi()
// Work only if compiler supports C++11 or above.
#include 
#include 
using namespace std;

int main()
{
	string str1 = "45";
	string str2 = "3.14159";
	string str3 = "31337 geek";

	int myint1 = stoi(str1);
	int myint2 = stoi(str2);
	int myint3 = stoi(str3);

	cout << "stoi(\"" << str1 << "\") is "
		<< myint1 << '\n';
	cout << "stoi(\"" << str2 << "\") is "
		<< myint2 << '\n';
	cout << "stoi(\"" << str3 << "\") is "
		<< myint3 << '\n';

	return 0;
}

Output:

stoi("45") is 45
stoi("3.14159") is 3
stoi("31337 geek") is 31337 

atoi() : The atoi() function takes a character array or string literal as an argument and returns its value. Following is a simple implementation:

C

#include 
#include 
int main()
{

	char* str1 = "42";
	char* str2 = "3.14159";
	char* str3 = "31337 geek";

	int myint1 = atoi(str1);
	int myint2 = atoi(str2);
	int myint3 = atoi(str3);

	printf("stoi(%s) is %d \n", str1, myint1);
	printf("stoi(%s) is %d \n", str2, myint2);
	printf("stoi(%s) is %d \n", str3, myint3);

	// This Code is Contributed by Harshit Srivastava
	return 0;
}

Output:

atoi("42") is 42
atoi("3.14159") is 3
atoi("31337 geek") is 31337 

stoi() vs atoi()

• atoi() is a legacy C-style function. stoi() is added in C++ 11. 
   
• atoi() works only for C-style strings (character array and string literal), stoi() works for both C++ strings and C style strings
• atoi() takes only one parameter and returns integer value.

int atoi (const char * str); 

• stoi() can take upto three parameters, the second parameter is for starting index and third parameter is for base of input number.

int stoi (const string&  str, size_t* index = 0, int base = 10); 

Exercise 
Write your own atof() that takes a string (which represents a floating-point value) as an argument and returns its value as double.