how to convert string to int c++

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11-03-2025

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Converting strings to integers is a common task in C++ programming. Whether you're reading data from a file, processing user input, or working with external libraries, you'll often need to transform string representations of numbers into their integer equivalents for calculations or other operations. This article explores several methods to achieve this conversion safely and efficiently.

Method 1: Using std::stoi, std::stol, std::stoll

The simplest and most recommended approach is using the C++ standard library functions: std::stoi, std::stol, and std::stoll. These functions handle the conversion from string to integer, long, and long long, respectively.

std::stoi (string to integer): Converts a string to an int.

#include <iostream>
#include <string>
#include <stdexcept>

int main() {
  std::string str = "12345";
  try {
    int num = std::stoi(str);
    std::cout << "Converted integer: " << num << std::endl;
  } catch (const std::invalid_argument& e) {
    std::cerr << "Invalid argument: " << e.what() << std::endl;
  } catch (const std::out_of_range& e) {
    std::cerr << "Out of range: " << e.what() << std::endl;
  }
  return 0;
}

std::stol (string to long): Converts a string to a long.

#include <iostream>
#include <string>
#include <stdexcept>

int main() {
    std::string str = "1234567890123456789"; // A larger number
    try {
        long num = std::stol(str);
        std::cout << "Converted long: " << num << std::endl;
    } catch (const std::invalid_argument& e) {
        std::cerr << "Invalid argument: " << e.what() << std::endl;
    } catch (const std::out_of_range& e) {
        std::cerr << "Out of range: " << e.what() << std::endl;
    }
    return 0;
}

std::stoll (string to long long): Converts a string to a long long. Use this for very large numbers.

Important Considerations:

• Error Handling: Always wrap these functions in a try-catch block to handle potential exceptions:
  • std::invalid_argument: Thrown if the string cannot be converted to a number (e.g., it contains non-numeric characters).
  • std::out_of_range: Thrown if the converted number is outside the range of the target integer type.
• Base Specification: These functions can also handle different number bases (e.g., hexadecimal, octal). See the C++ documentation for details.

Method 2: Using std::stringstream

std::stringstream offers a more flexible approach, particularly useful when dealing with more complex input strings that might contain other characters besides the number.

#include <iostream>
#include <string>
#include <sstream>

int main() {
  std::string str = "12345abc"; //String with non-numeric characters
  std::stringstream ss(str);
  int num;
  ss >> num; //Extract the number from the stringstream

  if (ss.fail()) {
    std::cerr << "Conversion failed!" << std::endl;
  } else {
    std::cout << "Converted integer: " << num << std::endl;
  }

  return 0;
}

This method extracts the number from the beginning of the string. Any characters after the number are ignored. If the string starts with non-numeric characters, the conversion will fail.

Method 3: Manual Conversion (Less Recommended)

While possible to manually convert character by character, this is generally less efficient and more error-prone than using the standard library functions. It's advisable to avoid this unless you have very specific constraints.

Choosing the Right Method

For most cases, std::stoi, std::stol, or std::stoll are the preferred methods due to their simplicity, efficiency, and built-in error handling. std::stringstream provides more flexibility for parsing complex input strings. Avoid manual conversion unless absolutely necessary. Remember to always handle potential exceptions to ensure your program's robustness.