How to convert string (string) to int (integer) in C++

When you code in C++, sometimes you need to convert one data type to another.

This article will introduce two commonly used methods to convert strings to integers using C++.

Before learning methods, you must first understand the data types of C++.

1. Data types in C++

The C++ programming language has some built-in data types:

• int, for integers (e.g. 10, 150)
• double, for floating point numbers (e.g. 5.0, 4.5)
• char, for a single character (e.g. "D", "!")
• string, for character sequences (e.g. "Hello")
• bool, for boolean values ​​(true or false)

C++ is a strongly typed programming language, which means that when you create a variable, you must explicitly declare what type of value will be stored in it.

2. How to convert string to integer

Method 1: Use the stoi() function to convert the string into an integer. This is a valid approach and works with newer versions of C++, introduced starting with C++11. It accepts a string as input and returns its integer form as output.

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

int main() {
   string str = "7";
   cout << "我是一个字符串 " << str << endl;

   int num = stoi(str);
   
   cout << "我是一个整数 " << num << endl;
}

Output result:

I am a string 7

I am an integer 7

Method 2: Use the stringstream class to convert the string into an integer. This approach mainly works with older versions of C++. It does this by inputting and outputting strings.

First, you need to add #include <sstream> at the top of the program to include the sstream library.

Then create a stringstream object that holds the value of the string you want to convert to an integer and use it during the conversion.

You can use <<operators to extract strings from string variables.

Finally, >>the newly converted int value is entered into the int variable using the operator.

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

using namespace std;

int main() {
   stringstream ss; 
   string str = "7";
   int num;
   
   ss << str;
   ss >> num;
   
   cout << num << endl;
}

Now that you know two simple ways to convert a string to an integer in C++, you can try some more.