The **`atol()`** function in C++ is used to convert a string to an integer data-type `long int`. `atol()` interprets the contents of the string as an integer.

# Library
To use the `atol()` function, include the following library:
```C
#include <cstdlib>
```
# Declaration
The `atol()` function can be declared as:
```C
long int atol(const char* str);
```
* `str`: The string to be converted to an integer.

# Return value
The `atol()` function returns a `long int` type variable that is the integer interpretation of the string `str`.

>Note: The `atol()` function returns 0 if:
>* `str` is empty.
>* `str` contains only whitespace characters.
>* The sequence of non-whitespace characters in `str` is not a valid integer.
>* Non-numeric characters are placed before numeric characters.


# Examples
## Example 1
Consider the code snippet below, which demonstrates the use of the `atol()` function:


#include <cstdlib>
#include <iostream>
using namespace std;

int main()
{
	
	char str1[] = "123"; 
	char str2[] = "-123"; 
	char str3[] = "12end with words";

	long int asint1 = atol(str1); 
	cout<<"atol( "<<str1<<" ) = "<<asint1<<endl;

	long int asint2 = atol(str2); 
	cout<<"atol( "<<str2<<" ) = "<<asint2<<endl;

	long int asint3 = atol(str3); 
	cout<<"atol( "<<str3<<" ) = "<<asint3<<endl;
	
	return 0;
}


## Explanation
Three strings, `str1`, `str2`, and `str3`, are declared in *lines 8-10*. The `atol()` function is used in *line 12*, *line 15*, and *line 18* to convert `str1`, `str2`, and `str3` to integers, respectively.

## Example 2
Consider the code snippet below, which demonstrates when the `atol()` function returns 0:

#include <cstdlib>
#include <iostream>
using namespace std;

int main()
{
	
	char str1[] = ""; 
	char str2[] = " \t "; 
	char str3[] = "abc"; 
	char str4[] = "start with words12";

	long int asint1 = atol(str1); 
	cout<<"atol( "<<str1<<" ) = "<<asint1<<endl;

	long int asint2 = atol(str2); 
	cout<<"atol( "<<str2<<" ) = "<<asint2<<endl;

    long int asint3 = atol(str3); 
	cout<<"atol( "<<str3<<" ) = "<<asint3<<endl;

	long int asint4 = atol(str4); 
	cout<<"atol( "<<str4<<" ) = "<<asint4<<endl;
	
	return 0;
}


## Explanation
Four strings, `str1`, `str2`, `str3`, and `str4`, are declared in *lines 8-11*.
* *line 13*: The `atol()` function is used to convert `str1` to an integer. The `atol()` function returns 0, as `str1` is empty.
* *line 16*: The `atol()` function is used to convert `str2` to an integer. The `atol()` function returns 0, as `str2` only contains whitespace characters.
* *line 19*: The `atol()` function is used to convert `str3` to an integer. The `atol()` function returns 0, as non-whitespace characters of `str3` do not represent a valid integer.
* *line 22*: The `atol()` function is used to convert `str4` to an integer. The `atol()` function returns 0, as non-numeric characters are placed before numeric characters of `str4`.

What is atol() in C++?

The atol() function in C++ converts a string to a long int, returning 0 in invalid cases.