C++ String strlen() Function

What You’ll Learn

The strlen() function answers a basic question: how long is this C-string? It counts characters from the start until (but not including) the null terminator '\0'. Knowing length helps you loop over text, validate input, and size buffers before calling functions like strcpy().

Definition and Usage

In C++, strlen() is declared in <cstring>. It accepts a pointer to a null-terminated C-string and returns the number of bytes (for plain char text, the number of characters) before the terminating '\0'.

String literals like "Hello, C++!" automatically include a hidden null terminator at the end. strlen sees 11 visible characters and stops—it never counts the twelfth byte that holds '\0'.

📝 Syntax

Function signature and a typical call:

#include <cstring>

size_t strlen(const char* str);

// Example:
std::size_t len = strlen("Hello, C++!");
// len == 11

Parameters

• str — pointer to a null-terminated C-string (literal, char array, or const char*).

Return Value

Returns a size_t equal to the number of characters before the first '\0'. An empty string "" has length 0.

Examples Gallery

Compile with g++ strlen.cpp -std=c++17 -o strlen. Each example shows a practical way to use or relate to strlen().

#include <iostream>
#include <cstring>

int main() {
    const char* str = "Hello, C++!";

    std::size_t length = std::strlen(str);

    std::cout << "String: \"" << str << "\"\n";
    std::cout << "Length: " << length << " characters\n";

    return 0;
}

String: "Hello, C++!"
Length: 11 characters

#include <iostream>
#include <cstring>

int main() {
    const char* empty = "";
    char buffer[1] = { '\0' };

    std::cout << "Literal \"\":  " << std::strlen(empty) << "\n";
    std::cout << "Char array:   " << std::strlen(buffer) << "\n";

    return 0;
}

Literal "":  0
Char array:   0

#include <iostream>
#include <cstring>

int main() {
    const char* source = "CodeToFun";

    std::size_t chars = std::strlen(source);
    std::size_t bytes = chars + 1; // +1 for '\0'

    char* copy = new char[bytes];
    std::strcpy(copy, source);

    std::cout << "Chars: " << chars << ", bytes allocated: " << bytes << "\n";
    std::cout << "Copy:  " << copy << "\n";

    delete[] copy;
    return 0;
}

Chars: 9, bytes allocated: 10
Copy:  CodeToFun

#include <iostream>
#include <string>
#include <cstring>

int main() {
    const char* cstr = "Tutorial";
    std::string cppStr = "Tutorial";

    std::cout << "strlen:           " << std::strlen(cstr) << "\n";
    std::cout << "std::string size: " << cppStr.size() << "\n";
    std::cout << "Same length?       "
              << std::boolalpha
              << (std::strlen(cstr) == cppStr.size()) << "\n";

    return 0;
}

strlen:           8
std::string size: 8
Same length?       true

#include <iostream>
#include <cstring>

int main() {
    const char* word = "strlen";

    std::size_t len = std::strlen(word);

    std::cout << "Characters: ";
    for (std::size_t i = 0; i < len; ++i) {
        std::cout << word[i];
        if (i + 1 < len) {
            std::cout << '.';
        }
    }
    std::cout << "\nTotal: " << len << "\n";

    return 0;
}

Characters: s.t.r.l.e.n
Total: 6

🚀 Common Use Cases

• Buffer allocation — compute strlen + 1 before strcpy or malloc.
• Input validation — enforce minimum or maximum name lengths.
• Loop bounds — iterate over each character in a C-string.
• Formatting output — pad or align text based on length.
• Interfacing with C APIs — many functions expect you to know string sizes.

📝 Notes

• The argument must be a null-terminated C-string—otherwise behavior is undefined.
• strlen is from the C library (<cstring>), not the C++ STL containers.
• Do not pass a std::string object directly; use s.c_str() if you must call strlen on its content.
• Time complexity is O(n) where n is the string length.
• For binary data that may contain zero bytes, use a length-aware type instead of C-strings.

⚡ Optimization

Cache the result when you need the length more than once. Avoid writing for (i = 0; i < strlen(s); ++i) because strlen rescans from the start on every comparison. Prefer range-for over std::string, or loop until '\0' when working with C-strings in hand-written parsers.

💡 Best Practices