How can I convert a std::string to int?

Question

Just have a quick question. I\'ve looked around the internet quite a bit and I\'ve found a few solutions but none of them have worked yet. Looking at converting a string to

Answer 1

My Code:

#include <iostream>
using namespace std;

int main()
{
    string s="32";  //String
    int n=stoi(s);  //Convert to int
    cout << n + 1 << endl;

    return 0;
}

Answer 2

To be more exhaustive (and as it has been requested in comments), I add the solution given by C++17 using std::from_chars.

std::string str = "10";
int number;
std::from_chars(str.data(), str.data()+str.size(), result);

If you want to check whether the conversion was successful:

std::string str = "10";
int number;
auto [ptr, ec] = std::from_chars(str.data(), str.data()+str.size(), result);
assert(ec == std::errc{});
// ptr points to chars after read number

Moreover, to compare the performance of all these solutions, see the following quick-bench link: https://quick-bench.com/q/GBzK53Gc-YSWpEA9XskSZLU963Y

(std::from_chars is the fastest and std::istringstream is the slowest)

Answer 3

To convert from string representation to integer value, we can use std::stringstream.

if the value converted is out of range for integer data type, it returns INT_MIN or INT_MAX.

Also if the string value can’t be represented as an valid int data type, then 0 is returned.

#include 
#include 
#include 

int main() {

    std::string x = "50";
    int y;
    std::istringstream(x) >> y;
    std::cout << y << '\n';
    return 0;
}

Output: 50

As per the above output, we can see it converted from string numbers to integer number.

Source and more at string to int c++

Answer 4

Admittedly, my solution wouldn't work for negative integers, but it will extract all positive integers from input text containing integers. It makes use of numeric_only locale:

int main() {
        int num;
        std::cin.imbue(std::locale(std::locale(), new numeric_only()));
        while ( std::cin >> num)
             std::cout << num << std::endl;
        return 0;
}

Input text:

 the format (-5) or (25) etc... some text.. and then.. 7987...78hjh.hhjg9878

Output integers:

 5
25
7987
78
9878

The class numeric_only is defined as:

struct numeric_only: std::ctype<char> 
{
    numeric_only(): std::ctype<char>(get_table()) {}

    static std::ctype_base::mask const* get_table()
    {
        static std::vector<std::ctype_base::mask> 
            rc(std::ctype<char>::table_size,std::ctype_base::space);

        std::fill(&rc['0'], &rc[':'], std::ctype_base::digit);
        return &rc[0];
    }
};

Complete online demo : http://ideone.com/dRWSj

Answer 5

use the atoi function to convert the string to an integer:

string a = "25";

int b = atoi(a.c_str());

http://www.cplusplus.com/reference/clibrary/cstdlib/atoi/

Answer 6

It's probably a bit of overkill, but boost::lexical_cast<int>( theString ) should to the job quite well.