C++ String::stol() function

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The C++ std::string::stol() function is used to convert a string to a long integer. It takes a string as input and convert it to a long type, with an optional base parameter for numeric conversion. Additionally, it take a pointer to a size_t to store the nuber of characters processed. If the conversion is failed, it throws an invalid_argument exception.

Syntax

Following is the syntax for std::string::stol() function.

long stol (const string&  str, size_t* idx = 0, int base = 10);
long stol (const wstring& str, size_t* idx = 0, int base = 10);

Parameters

• str − It indicates the string object with the representation of a integral number.
• idx − It indicates the pointer to an object of type size_t, whose value is set by the function to position of the next character in str after the numerical value.
• base − It indicates the numerical base that determines the valid character and their interpretation.

Return value

It returns a string as the value into a long integer.

Example 1

Following is the basic example to convert a decimal string into a long int using C++.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string decimal = "123456";
   long int dec_num = stol(decimal);
   cout << "The long integer = " << dec_num << endl;
   return 0;
}

Output

If we run the above code it will generate the following output.

The long integer = 123456

Example 2

In this example, we are passing a hexadecimal string to convert into a long int.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string hex = "987654F";
   long int hex_num = stol(hex, nullptr, 16);
   cout << "The long integer = " << hex_num << endl;
   return 0;
}

Output

If we run the above code it will generate the following output.

The long integer = 159868239

Example 3

This program extracts a long integer from a string containing alphanumeric characters, stopping at the first non-numeric character, and prints the integer to the complier.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string mixed = "10000px";
   size_t pos;
   long int number = stol(mixed, & pos);
   cout << "The long integer = " << number << endl;
   return 0;
}

Output

Following is the output of the above code.

The long integer = 100000                

Example 4

This program converts a base-24 string 64B to its equivalent long integer value.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string base24 = "64B";
   long int base24_num = stol(base24, nullptr, 24);
   cout << "The long integer = " << base24_num << endl;
   return 0;
}

Output

Following is the output of the above code.

The long integer = 3563