Convert String to Integer in C++

In C++, you can convert a string to an integer using several methods. In this tutorial, we will explore some of the most common methods to perform this conversion.

Syntax

#include <string>
#include <cstdlib>

// Using stoi:
int i = std::stoi("123");

// Using atoi:
int i = std::atoi("123");

// Using stringstream:
int i;
std::stringstream("123") >> i;

Example

Using stoi

#include <iostream>
#include <string>

using namespace std;

int main() {
    string str = "123";
    int num = stoi(str);
    cout << "The integer is: " << num << endl;
    return 0;
}

Using atoi

#include <iostream>
#include <cstdlib>

using namespace std;

int main() {
    char str[] = "123";
    int num = atoi(str);
    cout << "The integer is: " << num << endl;
    return 0;
}

Using stringstream

#include <iostream>
#include <sstream>

using namespace std;

int main() {
    string str = "123";
    int num;
    stringstream ss(str);
    ss >> num;
    cout << "The integer is: " << num << endl;
    return 0;
}

Output

The integer is: 123

Explanation

In all three examples, we start with a string containing an integer value. We then use different methods to convert this string to an integer.

std::stoi is a C++11 function that accepts a string as input and returns an integer.

std::atoi is a function in the C standard library that converts a string to an integer.

std::stringstream is a class in the C++ standard library that allows us to read from and write to strings as if they were streams.

Use

Converting a string to an integer is useful when we need to perform arithmetic operations on a string that contains numerical data. We can also use this conversion to validate user input, as we can easily check if the input is a valid integer by attempting to convert it to one.

Important Points

• C++ provides several methods to convert a string to an integer.
• These methods include std::stoi, std::atoi, and std::stringstream.
• Converting a string to an integer is useful when we need to perform arithmetic operations on a string that contains numerical data.
• We can also use this conversion to validate user input.

Summary

In summary, we can convert a string to an integer in C++ using several methods. These methods include std::stoi, std::atoi, and std::stringstream. Converting a string to an integer is useful when we need to perform arithmetic operations on a string that contains numerical data or validate user input.