Convert a String to an int in C++

Converting a string to an integer in C++ can be simple, but choosing the right method, whether stoi(), stringstream, or manual parsing, can confuse the user as they have different performance, safety, and error handling. Many C++ developers, from beginners to experienced coders, have to find which approach is the most reliable or error-free. In this guide, we will explore all the best and most efficient ways to convert a C++ string to an int, compare their pros and cons, and provide real-world code examples in detail.

Table of Contents:

• How to Convert a String to an Integer
  • Using stoi() Function
  • Using atoi() Function
  • Using stringstream
  • Using sscanf() Function
  • Using a For Loop (Manual Method)
  • Using strtol() Function
• Conclusion

How to Convert a String to an Integer.

In C++, converting from a string to an integer is a very basic thing. These methodologies are useful, especially when you are reading integer elements from user input, files, or different text-based information sources. We can convert the strings to integers in multiple ways. Both of these methods serve different use cases as performance, input reliability, etc.

1. Using stoi() Function

In C++, the stoi() function provides a simple and efficient way to convert a string to an int datatype.  It is declared by using the header <string> and belongs to the std namespace.

Example:

Output: 

In the above code, after initializing the <string> header, the string declaration follows. The string str = “123” declares the number(123) as a string by using double quotes. Now, using stoi, it converts the string into a number; using cout, the number is displayed on your console. 

2. Using atoi() Function

The atoi() function is a C-style function that converts a C-string to an integer. To declare this function in C++, we have to use the <cstdlib> header in our program. 

Example: 

Output: 

In the above program, we use atoi() to change the C-style string “456” to an integer. The result is printed with cout. 

Difference between stoi() and atoi() 

#include <cstdlib>
#include <iostream>
using namespace std;

int main() {
    const char* str = "123";
    int num = atoi(str);
    cout << num;
    return 0;
}
// Output: 123
                    

#include <string>
#include <iostream>
using namespace std;

int main() {
    string str = "123";
    int num = stoi(str);
    cout << num;
    return 0;
}
// Output: 123
                    

3. Using stringstream

The stringstream class is declared in the header file as <sstream>, which is part of the C++ standard library. Stringstream guarantees a robust and effective approach to transforming the string into any separate data type. It acts as an alternative to atoi() and is extremely helpful when dealing with formatted input.

Syntax: 
std::stringstream ss;
ss << string_value;
ss >> int_variable;
Example: 

Output: 

In the above program, we use the header to declare the string stream. Now in this program, the string”789”, which got stored in the variable str, is type casted to an integer. The stringstream object ss is created and initialized with str. An integer value is retrieved from the stream into the variable num using the operator >>. Finally, the converted number is printed using the cout.

4. Using sscanf() Function

The sscanf() is a member of the C standard library, which is declared for taking data formatted like a C-string. sscanf() works like printf(); however, instead of reading from the user, it reads from a string.

Example: 

Output: 

Here, the above code shows how to convert a C-style string “321” to an integer using the sscanf() method provided by the standard library in C. It saves the string in the char array str. ” num = sscanf( str, “%d” ); Then the program prints the converted number. cout will show the output on the console.

5. Using a For Loop (Manual Method)

We can convert the string into only digits by using a manual method for-loop, which generates a digit in a step-by-step approach.  This method is very efficient for learning how the conversion happens between the data types. 

Example: 

Output: 

The program demonstrates how to convert the string “234” into an integer using a loop manually. You start with num = 0, then iterate through the digits in the string, converting it to an int by subtracting a character (‘0’) from the string. It computes the value using num = num * 10 + (c – ‘0’). After the loop, it prints the last number, 234. This illustrates how the internal operation of a string to an integer conversion works.

6. Using strtol() Function

Unlike atoi(), this strtlol() function is more robust and efficient in error checking. This function is used to detect invalid input or out-of-range values, which makes it safer and more reliable for use in real-world applications. 

Example: 

Output: 

The following program converts the string “987” to an int using strtol(). At first, it takes the string, a pointer end from where to know where the conversion stops, and the base (10). This result is also stored in num and printed. Unlike atoi(), the strtol() function allows some very minimal error checking via the end pointer.

Conclusion

There are several ways to convert a string to an int in modern C++ code. The stoi() function is a simple and safe approach. Functions like atoi() and sscanf() are legacy methods and don’t handle errors gracefully. For production applications, you might use strtol() or stringstream. It helps you understand all the methods better and ultimately write better and more reliable code.

Useful Resources:

• C++ Data Types
• STL (Standard Template Library) in C++
• Tokenizing a String in C++
• Frequently Asked C++ Programs

Convert a String to an int in C++ – FAQs