How can I convert a std::string to int in C++?

To convert a std::string to an int in C++, there are several methods, each with its own use cases and error-handling strategies. Below is a detailed explanation with examples:

1. Using std::stoi() (C++11 and later)

The std::stoi() function (from <string>) converts a string to an integer and throws exceptions for invalid inputs.

Basic Example:

#include <string>
#include <iostream>

int main() {
    std::string str = "12345";
    try {
        int num = std::stoi(str);
        std::cout << "Converted: " << 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;
}

Check Entire String Validity:

Use the optional size_t* pos parameter to ensure the entire string is parsed:

std::string str = "123abc";
size_t pos;
try {
    int num = std::stoi(str, &pos);
    if (pos != str.size()) {
        std::cout << "Invalid characters after number!" << std::endl;
    } else {
        std::cout << "Valid: " << num << std::endl;
    }
} catch (...) { /* Handle exceptions */ }

2. Using std::istringstream (Works in Older C++ Standards)

Leverage string streams (<sstream>) to parse the string and validate it fully.

Example:

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

int main() {
    std::string str = "  6789"; // Leading whitespace allowed
    std::istringstream iss(str);
    int num;

    // Extract the integer and check for trailing characters
    if (iss >> num) {
        char remaining;
        if (iss >> remaining) { // Check leftover characters
            std::cout << "Invalid trailing characters!" << std::endl;
        } else {
            std::cout << "Converted: " << num << std::endl;
        }
    } else {
        std::cout << "Conversion failed!" << std::endl;
    }
    return 0;
}

3. Using C Functions (strtol() or atoi())

For compatibility with older codebases or C-style validation.

Example with strtol() (Robust Error Handling):

#include <cstdlib>
#include <string>
#include <iostream>
#include <cerrno> // For errno

int main() {
    std::string str = "2147483648"; // Exceeds int range
    char* end;
    const char* cstr = str.c_str();
    long num = std::strtol(cstr, &end, 10); // Base 10

    if (cstr == end) {
        std::cout << "No digits found!" << std::endl;
    } else if (*end != '\0') {
        std::cout << "Invalid trailing characters!" << std::endl;
    } else if (errno == ERANGE || num < INT_MIN || num > INT_MAX) {
        std::cout << "Out of int range!" << std::endl;
    } else {
        int result = static_cast<int>(num);
        std::cout << "Converted: " << result << std::endl;
    }
    return 0;
}

Example with atoi() (Simple but Limited):

#include <cstdlib>
#include <string>
#include <iostream>

int main() {
    std::string str = "123";
    int num = std::atoi(str.c_str());
    std::cout << "Converted: " << num << std::endl;
    return 0;
}

Note: atoi() returns 0 for invalid inputs (e.g., "abc"), making it unreliable for error checking.

Key Differences

Method	Pros	Cons
std::stoi()	Simple, robust error handling	Requires C++11, throws exceptions
std::istringstream	Works in older C++ standards	Verbose, limited error messages
strtol()	Precise control, C compatible	Manual error checking, returns long
atoi()	Simple syntax	No error handling, deprecated in C++

Handling Edge Cases

1. Leading/Trailing Whitespace:

• std::stoi() and std::istringstream ignore leading whitespace.
• Use pos or stream checks to detect trailing characters.

1. Hexadecimal/Binary Strings:

• Use std::stoi(str, nullptr, 16) for hex or std::stoi(str, nullptr, 2) for binary.

1. Overflow/Underflow:

• std::stoi() throws std::out_of_range for values exceeding int limits.
• strtol() sets errno = ERANGE for overflow.

Summary

• For Modern C++: Use std::stoi() with exception handling.
• For Legacy Code: Use std::istringstream or strtol().
• Avoid atoi(): It lacks reliable error detection.

Choose the method based on your project’s C++ version and error-handling requirements.