How to Parse Int From String in C++

  1. Use the std::stoi Function to Parse Int From String
  2. Use the std::from_chars Function to Parse Int From String

This article will explain several methods of how to parse an int from the string in C++.

Use the std::stoi Function to Parse Int From String

The stoi function is part of the string library defined in the string header, and it can be utilized to convert string values to different numeric types. std::stoi, std::stol and std::stoll are used for signed integer conversion. The stoi function takes a single string object as a mandatory argument, but the programmer may also specify the address to store the integer and the number base in which the input string has to be processed.

The following example demonstrates multiple use cases of the stoi function. Note that stoi can handle leading white spaces in the string, but any other character will cause a std::invalid_argument exception.

#include <iostream>
#include <string>
#include <charconv>

using std::cout; using std::cin;
using std::endl; using std::string;
using std::stoi;

int main(){
    string s1 = "333";
    string s2 = "-333";
    string s3 = "333xio";
    string s4 = "01011101";
    string s5 = "    333";
    string s6 = "0x33";

    int num1, num2, num3, num4, num5, num6;

    num1 = stoi(s1);
    num2 = stoi(s2);
    num3 = stoi(s3);
    num4 = stoi(s4, nullptr, 2);
    num5 = stoi(s5);
    num6 = stoi(s6, nullptr, 16);

    cout << "num1: " << num1 << " | num2: " << num2 << endl;
    cout << "num3: " << num3 << " | num4: " << num4 << endl;
    cout << "num5: " << num5 << " | num6: " << num6 << endl;

    return EXIT_SUCCESS;
}

Output:

num1: 333 | num2: -333
num3: 333 | num4: 93
num5: 333 | num6: 51

Use the std::from_chars Function to Parse Int From String

As an alternative, the from_chars function from the utilities library can parse int values. It’s been part of the standard library since the C++17 version and is defined in the <charconv> header file.

Much different from the stoi, from_chars operates on the ranges of characters, not being aware of the object length and borders. Thus, the programmer should specify the start and end of the range as the first two arguments. The third argument is the int variable to which the converted value will be assigned.

Mind though, from_chars can only handle the leading - sign in the input string; hence it prints garbage values of num5 and num6 in the following example.

#include <iostream>
#include <string>
#include <charconv>

using std::cout; using std::cin;
using std::endl; using std::string;
using std::stoi; using std::from_chars;

int main(){
    string s1 = "333";
    string s2 = "-333";
    string s3 = "333xio";
    string s4 = "01011101";
    string s5 = "    333";
    string s6 = "0x33";

    int num1, num2, num3, num4, num5, num6;

    from_chars(s1.c_str(), s1.c_str() + s1.length(), num1);
    from_chars(s2.c_str(), s2.c_str() + s2.length(), num2);
    from_chars(s3.c_str(), s3.c_str() + s3.length(), num3);
    from_chars(s4.c_str(), s4.c_str() + s4.length(), num4, 2);
    from_chars(s5.c_str(), s5.c_str() + s5.length(), num5);
    from_chars(s6.c_str(), s6.c_str() + s6.length(), num6);

    cout << "num1: " << num1 << " | num2: " << num2 << endl;
    cout << "num3: " << num3 << " | num4: " << num4 << endl;
    cout << "num5: " << num5 << " | num6: " << num6 << endl;

    return EXIT_SUCCESS;
}

Output:

num1: 333 | num2: -333
num3: 333 | num4: 93
num5: -1858679306 | num6: 0

Related Article - C++ Int

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  • How to Convert Int to a String in C++
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