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The C++ std::string::stoi() function is used to convert a string to an integer (int). This function analyse the initial characters of the string as an integer and stops at the first non-numeric character. If the conversion is successful, it return the integer value, otherwise it throws an invalid_argument exception.

Syntax

Following is the syntax for std::string::stoi() function.

int stoi (const string&  str, size_t* idx = 0, int base = 10);
int stoi (const wstring& str, size_t* idx = 0, int base = 10);

Parameters

This function has two parameters as described below.

• str − It indicates the string object with the representation of a integral number.
• idx − It indicates the pointer to an object of type size_t, whose value is set by the function to position of the next character in str after the numerical value.
• base − It indicates the numerical base that determines the valid character and their interpretation.

Return value

It returns a integer value of the parsed string.

Example 1

Following is the basic example for the basic conversion to demonstrate the string::stoi using C++.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string s = "42";
   int num = stoi(s);
   cout << num << endl;
   return 0;
}

Output

If we run the above code it will generate the following output.

42

Example 2

In this example, we are handling Hexadecimal and Binary strings for integer conversion.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string hexStr = "2A";
   string binStr = "101010";
   int numHex = stoi(hexStr, nullptr, 16);
   int numBin = stoi(binStr, nullptr, 2);
   cout << numHex << ", " << numBin << endl;
   return 0;
}

Output

If we run the above code it will generate the following output.

42, 42

Example 3

In the below example, we are extracting numbers from mixed strings.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string mixedStr = "Year2024";
   int year = stoi(mixedStr.substr(4));
   cout << year << endl;
   return 0;
}

Output

Following is the output of the above code.

2024                 

Example 4

Following is an another example to demonstrate the string:stoi() function. Here the string is passed but that does not contains any valid digit i. e. the numeric value. so, it raises an invalid_argument exception.

#include <iostream>
#include <string>
using namespace std;
int main() {
   string invalidStr = "Tutorialspoint";
   try {
      int num = stoi(invalidStr);
   } catch (const std::invalid_argument & e) {
      cout << "Invalid input: " << e.what() << endl;
   }
   return 0;
}

Output

Following is the output of the above code.

Invalid input: stoi 

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