C++ String strcmp() Function

What You’ll Learn

The strcmp() function from <cstring> compares two C-strings in lexicographic order (like dictionary order based on character codes). It returns an integer whose sign tells you whether the first string is less than, equal to, or greater than the second—essential for sorting, searching, and command matching.

Definition and Usage

In C++, strcmp() walks both strings in parallel, comparing characters from first to last. When characters differ, it returns the difference of those character values (as an int). When one string ends before a difference is found, the shorter string is considered smaller.

You typically care about the sign of the result: zero means equal, negative means str1 sorts before str2, positive means str1 sorts after. The exact non-zero number is not standardized—do not rely on it being 1 or -1.

📝 Syntax

The standard library declaration:

int strcmp(const char* str1, const char* str2);

Parameters

• str1 — first null-terminated C-string.
• str2 — second null-terminated C-string.

Return Value

• 0 — strings are equal.
• < 0 — str1 is lexicographically less than str2.
• > 0 — str1 is lexicographically greater than str2.

Header

#include <cstring>

Examples Gallery

Compile with g++ strcmp.cpp -std=c++17 -o strcmp. Each example interprets the return sign correctly—never assume the exact non-zero value.

#include <iostream>
#include <cstring>

int main() {
    const char* str1 = "Hello, World!";
    const char* str2 = "Hello, C++!";

    int result = std::strcmp(str1, str2);

    if (result == 0) {
        std::cout << "The strings are equal." << std::endl;
    } else if (result < 0) {
        std::cout << "str1 is less than str2." << std::endl;
    } else {
        std::cout << "str1 is greater than str2." << std::endl;
    }

    return 0;
}

str1 is greater than str2.

#include <iostream>
#include <cstring>

int main() {
    const char* password = "secret123";
    const char* input    = "secret123";

    if (std::strcmp(input, password) == 0) {
        std::cout << "Access granted." << std::endl;
    } else {
        std::cout << "Access denied." << std::endl;
    }

    return 0;
}

Access granted.

#include <iostream>
#include <cstring>

void report(const char* a, const char* b) {
    int cmp = std::strcmp(a, b);
    std::cout << '"' << a << "\" vs \"" << b << "\": ";
    if (cmp < 0) std::cout << "a comes first\n";
    else if (cmp > 0) std::cout << "b comes first\n";
    else std::cout << "same\n";
}

int main() {
    report("apple", "banana");
    report("banana", "apple");
    report("cherry", "cherry");
    return 0;
}

"apple" vs "banana": a comes first
"banana" vs "apple": b comes first
"cherry" vs "cherry": same

#include <iostream>
#include <cstring>

int main() {
    const char* cmd = "help";

    if (std::strcmp(cmd, "quit") == 0) {
        std::cout << "Goodbye!" << std::endl;
    } else if (std::strcmp(cmd, "help") == 0) {
        std::cout << "Commands: help, quit" << std::endl;
    } else {
        std::cout << "Unknown command." << std::endl;
    }

    return 0;
}

Commands: help, quit

#include <iostream>
#include <string>

int main() {
    std::string a = "Hello, World!";
    std::string b = "Hello, C++!";

    if (a == b) {
        std::cout << "Equal" << std::endl;
    } else if (a.compare(b) < 0) {
        std::cout << "a is less than b" << std::endl;
    } else {
        std::cout << "a is greater than b" << std::endl;
    }

    return 0;
}

a is greater than b

🚀 Common Use Cases

• Sorting arrays of names — use strcmp as a comparator for qsort or custom sorts.
• Command-line parsing — match argv tokens to known subcommands.
• Configuration keys — compare option names read from files.
• Version strings — rough lexicographic ordering (not always semver-correct).
• C API interop — compare strings passed from libraries as const char*.

📝 Notes

• Comparison is case-sensitive—"A" and "a" differ.
• Both strings must be null-terminated; otherwise behavior is undefined.
• Do not use == on char* pointers to compare text—use strcmp.
• Only the sign of a non-zero result is portable; the exact value may vary.
• For comparing only the first n characters, use strncmp().

⚡ Optimization

strcmp() is heavily optimized in standard libraries and is appropriate for most comparisons. If you compare the same long strings repeatedly, consider caching lengths or using std::string objects to avoid repeated scans. For locale-aware sorting (accents, language rules), use strcoll() instead.

💡 Best Practices