C++ cstdlib strtol() - Convert String to Long

Introduction

The strtol() function in C++ is a part of the cstdlib library, designed to convert strings to a long integer (long). This utility is especially valuable when dealing with user input or parsing strings to extract numbers that are integral to the application's logic, such as configuration settings, numerical data processing, and more.

In this article, you will learn how to effectively use the strtol() function in various programming situations. Discover how this function helps in type conversion, handles invalid input, and works around certain edge cases to robustly integrate string-to-long conversion in your C++ applications.

Understanding strtol()

Basic Usage of strtol()

1. Include the cstdlib header to have access to strtol().

2. Prepare a string that represents a long integer.

3. Use strtol() to convert the string.

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   #include <cstdlib>
   char str[] = "1024";
   char *pEnd;
   long int li = strtol(str, &pEnd, 10);

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   Here, strtol() converts the string str into a long integer. The third argument 10 represents the base, which in this case is decimal. pEnd will point to the part of the string following the numeric value, which in this example should point to the string's null terminator as there are no characters after the number.

Handling Invalid Input

1. Check for numbers with non-numeric suffixes or prefixes.

2. Apply strtol() and examine the pointer to where the numeric conversion stopped.

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   char input[] = "1234abc";
   long int number = strtol(input, &pEnd, 10);
   if (*pEnd != '\0') {
       std::cout << "Conversion stopped at: " << pEnd << std::endl;
   }

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   Here, you attempt to convert input using strtol(). The function will convert the 1234 part and then stop at abc because it's not a valid numeric character, which you can then handle accordingly.

Working With Different Bases

1. Understand that strtol() can convert strings in various bases (from 2 to 36).

2. Convert a hexadecimal string using strtol().

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   char hexStr[] = "1A3F";
   long int hexNumber = strtol(hexStr, nullptr, 16);
   std::cout << "Decimal: " << hexNumber << std::endl;

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   This code snippet takes a hexadecimal number represented as a string ("1A3F") and converts it into a long integer using base 16. If the conversion is successful, it will print the decimal equivalent of the hexadecimal number.

Using strtol() Effectively

Error Handling

1. Use the errno from <cerrno> to detect overflows and underflows.

2. Test strtol() with a larger-than-long value.

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   #include <cerrno>
   errno = 0;  // reset errno to 0 before conversion attempt
   char largeNum[] = "999999999999999999999999999999";
   long int large = strtol(largeNum, &pEnd, 10);
   if (errno == ERANGE) {
       std::cout << "Overflow or underflow occurred.\n";
   }

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   In this section, if an overflow occurs due to the magnitude of the number in largeNum, strtol() sets errno to ERANGE. By checking errno, you can effectively manage such errors and take necessary actions like logging the error or prompting the user for a different input.

Conclusion

The strtol() function in C++ is an invaluable tool for converting strings to long integers in a variety of contexts. By understanding and handling special cases such as different numeric bases and invalid inputs, you enhance the robustness of your applications. Whether dealing with user inputs in a UI application or reading configuration files in a system program, strtol() provides the functionality needed to correctly interpret and use numeric values embedded in strings. Implement these strategies to ensure your code remains reliable and effective in handling string-to-number conversions.