How to Get a Substring in C++ with substr()

Pulling a smaller piece out of a string — a file extension, a username, the first word — is everyday work in C++. The substr() method handles all of it. Once you understand its two arguments, slicing strings becomes second nature.

The Basics of substr()

Call substr() on any std::string. It takes a starting position and a length, and returns a brand-new string with that slice:

#include <iostream>
#include <string>

int main() {
    std::string text = "Programming in C++";
    std::cout << text.substr(0, 11) << "\n";  // Programming
    std::cout << text.substr(15) << "\n";     // C++
    return 0;
}

Two things to internalise. First, positions start at 0, so position 0 is the P. Second, the second number is a length, not an end position — substr(0, 11) means “11 characters starting at 0.”

Grab Everything After a Position

Leave out the length and substr() returns everything from your starting position to the end of the string. Combine it with find() to slice at a specific character — here, pulling a file’s extension:

#include <iostream>
#include <string>

int main() {
    std::string path = "report.pdf";
    std::size_t dot = path.find('.');
    std::string extension = path.substr(dot + 1);
    std::cout << "Extension: " << extension << "\n";  // pdf
    return 0;
}

find('.') returns the index of the dot, and substr(dot + 1) takes everything after it. This “find a marker, then slice” pattern is the workhorse of simple text parsing.

A Practical Example: Splitting an Email

Combine the two ideas to split an email address into its user and domain parts at the @:

#include <iostream>
#include <string>

int main() {
    std::string email = "ada@example.com";
    std::size_t at = email.find('@');
    std::string user = email.substr(0, at);   // up to the @
    std::string domain = email.substr(at + 1); // after the @
    std::cout << "User: " << user << "\n";      // ada
    std::cout << "Domain: " << domain << "\n";  // example.com
    return 0;
}

substr(0, at) takes the characters before the @ (a length of at), and substr(at + 1) takes everything after it. No characters are copied twice, and the original email is left unchanged.

Avoiding the out_of_range Error

There’s one crash to watch for. If the starting position is larger than the string’s length, substr() throws a std::out_of_range exception and your program stops. A quick length check keeps you safe:

#include <iostream>
#include <string>

int main() {
    std::string s = "short";
    std::size_t pos = 10;
    if (pos <= s.length())
        std::cout << s.substr(pos) << "\n";
    else
        std::cout << "Position is out of range\n";
    return 0;
}

This matters most when pos comes from find(), which returns std::string::npos (a huge value) when it finds nothing. Always confirm find() succeeded before feeding its result into substr().

Quick Reference

Related Articles

• C++ String Handling — the std::string basics
• C++ String Contains a Substring — searching with find()
• C++ Split a String — break text into multiple pieces
• C++ Reverse a String — another common string task
• C++ getline: Read a Full Line of Input — get the strings you’ll slice

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