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As evident from its name, strlen() is a function defined in the cstring header file, which gives us the total number of characters in a string, stored in a c-style char array.

Signature of strlen() function

size_t strlen(const char *str);)

• str - is a string passed to this function, whose length needs to be found.
• This function returns an int value, which tells us total number of characters in a string which was passed to this function.

Note :

• While calculating the total length of a String, the character at the first index is counted as 1 and not 0, i.e. one-based index .
• The function strlen returns the total number of characters actually present in the char[] and not the total number of character it can hold.

strlen() example

In the upcoming example, we have initialized two char array with different strings. Next, we are calling strlen() function on each of these String objects to determine their length.

/* Program to find the total length of a String using strlen() */

#include<iostream>
#include<cstring>

using namespace std;

int main()
{
// First string 
char str1[10]= "01234567"; 	

cout<< "First string is : "<< str1 << "\n";
int length = strlen(str1);
cout<< "Length of first string is : " << length << "\n";


// Second string 
char str2[20]= "String Chapter";	

cout<< "Second string is : " << str2 << "\n";
length = strlen(str2);
cout<< "Length of second string is : " << length;

return 0;
}

Output is :

First String is 01234567
Length of first String is 8
Second String is String Chapter
Length of second String is 14

Program Analysis

• In this code above, calling strlen() function by passing the first string, returns int value 8, which is the total number of characters in the passed string.
• Next, on calling strlen() function by passing the second string has returned 14, i.e. the total number of characters in this string.

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Finding length of an empty string using strlen()

In the upcoming example, we have finding length of an empty string, which is created but not initialized with any value.

// Program to find the length of an empty string


#include<iostream>
#include<cstring>

using namespace std;

int main()
{
char str1[10] = ""; // An empty string 
 
cout<< "The elements of the first string : " << str1 << "\n";
int length = strlen(str1);
cout<< "Length of the first string is : " << length << "\n";


// Another way to create a string 
char str2[] = {'A'}; // Not an empty string 

cout<< "The elements of the second string : " << str2 << "\n";
length = strlen(str2);
cout<< "Length of the second string is : " << length;

return 0;
}

Output is :

The elements of the first string :
Length of the first string is : 0
The elements of the second string : A
Length of the second tring is : 1

Program Analysis

• In this code above, calling strlen() function on an empty String object, returns int value 0, because this string is created but not initialized with a value.
• Next, a string is initialized with a single character value and passed to strlen() function, which returns 1, i.e. the total number of characters in the string.

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