SysTutorials

Checking if a String Starts With Another in C++

· Eric Ma · updated April 12, 2026

C++20 introduced std::string::starts_with(), which is the most straightforward way to check if a string begins with a specific substring:

#include <iostream>
#include <string>

int main() {
    std::string text = "big string here...";

    std::cout << std::boolalpha;
    std::cout << text.starts_with("big") << std::endl;      // true
    std::cout << text.starts_with("small") << std::endl;    // false
    std::cout << text.starts_with("big string") << std::endl; // true

    return 0;
}

This is the preferred approach for modern C++ codebases. It’s clear, efficient, and readable.

Using std::string::compare() for C++11/C++14

If you’re working with C++11 or C++14, use compare() with the following signature:

int compare(size_type pos, size_type count, const basic_string& str) const;

The function returns 0 if the substring matches, and non-zero otherwise. Set pos to 0 and count to the length of the prefix you’re checking:

#include <iostream>
#include <string>

bool startswith(const std::string& str, const std::string& prefix) {
    return str.compare(0, prefix.length(), prefix) == 0;
}

int main() {
    std::cout << std::boolalpha;
    std::cout << startswith("big string here...", "big") << std::endl;      // true
    std::cout << startswith("big string here...", "small") << std::endl;    // false
    std::cout << startswith("big", "big string here") << std::endl;         // false

    return 0;
}

Note: compare() safely handles edge cases where the prefix is longer than the string — it returns non-zero without undefined behavior.

Using find() as an Alternative

You can also use find() with position 0, though it’s less idiomatic:

bool startswith(const std::string& str, const std::string& prefix) {
    return str.find(prefix) == 0;
}

This works but has a potential gotcha: if the prefix is empty, find() returns 0 (a match), which is technically correct behavior. However, find() is less efficient than compare() for this use case since it’s optimized for searching anywhere in the string.

Checking for Case-Insensitive Prefixes

For case-insensitive comparison, convert both strings to lowercase first:

#include <iostream>
#include <string>
#include <algorithm>

bool startswithi(std::string str, std::string prefix) {
    std::transform(str.begin(), str.end(), str.begin(), ::tolower);
    std::transform(prefix.begin(), prefix.end(), prefix.begin(), ::tolower);
    return str.starts_with(prefix);
}

int main() {
    std::cout << std::boolalpha;
    std::cout << startswithi("BIG String", "big") << std::endl;  // true
    std::cout << startswithi("Small String", "big") << std::endl; // false

    return 0;
}

For production code with performance requirements, consider using std::string_view to avoid creating temporary strings:

#include <iostream>
#include <string>
#include <string_view>
#include <algorithm>

bool startswithi(std::string_view str, std::string_view prefix) {
    if (prefix.size() > str.size()) return false;

    return std::equal(prefix.begin(), prefix.end(), 
                      str.begin(), 
                      [](unsigned char a, unsigned char b) {
                          return std::tolower(a) == std::tolower(b);
                      });
}

int main() {
    std::cout << std::boolalpha;
    std::cout << startswithi("BIG String", "big") << std::endl;
    std::cout << startswithi("Small String", "big") << std::endl;

    return 0;
}

Empty String Behavior

All methods handle empty prefix strings consistently — an empty prefix is considered to match any string:

std::string text = "anything";
std::cout << text.starts_with("") << std::endl;  // true

This is the correct and expected behavior.

Recommendation

Use std::string::starts_with() for C++20 and later projects. For older codebases, compare() is the reliable workhorse. Avoid find() for prefix checking unless you have a specific reason — it’s not semantically clear and performs unnecessary work.

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