How to Use substr in C++: Syntax and Examples

In C++, use the substr method of the std::string class to extract a substring. Call it like str.substr(start, length), where start is the starting index and length is how many characters to take.

Syntax

The substr method extracts a part of a string starting from a given position and for a specified length.

• start: The zero-based index where extraction begins.
• length: (Optional) Number of characters to extract. If omitted, extracts till the end of the string.

cpp

string substr (size_t start = 0, size_t length = npos) const;

Example

This example shows how to extract substrings from a string using substr. It prints parts of the original string based on start position and length.

cpp

#include <iostream>
#include <string>

int main() {
    std::string text = "Hello, world!";

    // Extract "Hello"
    std::string part1 = text.substr(0, 5);

    // Extract "world"
    std::string part2 = text.substr(7, 5);

    // Extract from index 7 to end "world!"
    std::string part3 = text.substr(7);

    std::cout << part1 << std::endl;
    std::cout << part2 << std::endl;
    std::cout << part3 << std::endl;

    return 0;
}

Output

Hello
world
world!

Common Pitfalls

Common mistakes when using substr include:

• Using a start index greater than the string length causes an out_of_range exception.
• Requesting a length that extends beyond the string end does not cause error; it returns up to the string's end.
• Remember that string indices start at 0, so the first character is at position 0.

cpp

#include <iostream>
#include <string>

int main() {
    std::string s = "Example";

    // Wrong: start index too large
    try {
        std::string wrong = s.substr(10, 2); // throws std::out_of_range
    } catch (const std::out_of_range& e) {
        std::cout << "Caught out_of_range error: " << e.what() << std::endl;
    }

    // Right: check length before calling substr
    if (s.size() > 10) {
        std::string safe = s.substr(10, 2);
    } else {
        std::cout << "Start index too large, skipping substr call." << std::endl;
    }

    return 0;
}

Output

Caught out_of_range error: basic_string::substr

Quick Reference

Summary tips for using substr:

• Use substr(start, length) to get a substring.
• If length is omitted, substring goes to the end.
• Check that start is within string length to avoid exceptions.
• Indices start at 0.

Key Takeaways

Use std::string::substr(start, length) to extract parts of a string in C++.

The start index must be within the string length to avoid exceptions.

If length is omitted, substr returns from start to the string's end.

String indices start at zero, so count carefully.

Always handle or check for out_of_range exceptions when using substr.