C++ strlen()

The strlen() function in C++ returns the length of the given C-string. It is defined in the cstring header file.


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

int main() {

  // initialize C-string
  char song[] = "We Will Rock You!";

// print the length of the song string cout << strlen(song);
return 0; } // Output: 17

strlen() Syntax

The syntax of the strlen() function is:

strlen(const char* str);

Here, str is the string whose length we need to find out, which is casted to a const char*.

strlen() Parameters

The strlen() function takes the following parameter:

  • str - pointer to the C-string (null-terminated string) whose length is to be calculated

strlen() Return Value

The strlen() function returns:

  • the length of the C-string (size_t)

strlen() Prototype

The prototype of strlen() as defined in the cstring header file is:

size_t strlen(const char* str);

Note: The returned length does not include the null character '\0'.

strlen() Undefined Behavior

The behavior of strlen() is undefined if:

  • there is no null character '\0' in the string i.e. if it is not a C-string

Example: C++ strlen()

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

int main() {
  char str1[] = "This a string";
  char str2[] = "This is another string";

// find lengths of str1 and str2 // size_t return value converted to int int len1 = strlen(str1); int len2 = strlen(str2);
cout << "Length of str1 = " << len1 << endl; cout << "Length of str2 = " << len2 << endl; if (len1 > len2) cout << "str1 is longer than str2"; else if (len1 < len2) cout << "str2 is longer than str1"; else cout << "str1 and str2 are of equal length"; return 0; }


Length of str1 = 13
Length of str2 = 22
str2 is longer than str1

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