C++ Convert int to string (and string to int): Complete Guide

Type conversions between numbers and strings are something you’ll do constantly in C++. You read a number from user input (which arrives as text), you build a filename with a number in it, you display a formatted message combining words and values. All of these require converting between int (or double) and string.

C++ has clean, modern functions for both directions. This tutorial covers all of them.

int to string: to_string()

The simplest way to convert any numeric type to a string is std::to_string(), available since C++11:

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

int main() {
    int n = 42;
    string s = to_string(n);
    cout << s << endl;         // "42"
    cout << s + " is the answer" << endl;  // "42 is the answer"
    return 0;
}

to_string() works with int, long, long long, float, double, and long double:

double price = 9.99;
string label = "$" + to_string(price);
cout << label << endl;  // "$9.990000"

Notice the trailing zeros — to_string() with doubles gives 6 decimal places by default. If you need more control over formatting, use stringstream (covered below).

string to int: stoi()

stoi() converts a std::string to an int. The name stands for string to int:

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

int main() {
    string s = "42";
    int n = stoi(s);
    cout << n + 1 << endl;  // 43
    return 0;
}

stoi() handles leading whitespace automatically:

int n = stoi("  100  ");  // n = 100, whitespace ignored

It stops at the first non-numeric character:

int n = stoi("42abc");  // n = 42, stops at 'a'

The stoi Family

C++11 provides a set of related functions for different numeric types:

#include <string>
using namespace std;

string s1 = "12345678901234";  // Too big for int
long long big = stoll(s1);    // Use stoll for large numbers

string s2 = "3.14159";
double pi = stod(s2);          // Use stod for decimals

Handling Errors with stoi

stoi() throws exceptions when conversion fails. You should handle them:

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

int main() {
    string input = "hello";

    try {
        int n = stoi(input);
        cout << n << endl;
    } catch (const invalid_argument& e) {
        cout << "Not a valid number: " << input << endl;
    } catch (const out_of_range& e) {
        cout << "Number too large for int" << endl;
    }

    return 0;
}

invalid_argument fires when the string doesn’t contain a number at all. out_of_range fires when the number is too big to fit in the target type.

This is particularly important when reading user input — users don’t always type what you expect.

The Old Way: atoi() (Avoid in New Code)

Before C++11, atoi() (from <cstdlib>) was the standard approach:

#include <cstdlib>
const char* s = "42";
int n = atoi(s);  // 42

The problem: when atoi() fails, it returns 0 — the same as successfully parsing "0". You can’t tell success from failure.

stoi() throws exceptions on failure, so you always know when something went wrong. Use stoi() in all new code.

Formatted Conversion: stringstream

When to_string() doesn’t give you enough control — like limiting decimal places — use stringstream:

#include <iostream>
#include <sstream>
#include <iomanip>
using namespace std;

int main() {
    double price = 9.5;

    ostringstream oss;
    oss << fixed << setprecision(2) << price;
    string formatted = oss.str();

    cout << "$" + formatted << endl;  // "$9.50"
    return 0;
}

fixed means don’t use scientific notation. setprecision(2) means 2 decimal places. oss.str() extracts the string from the stream.

You can also build complex strings by streaming multiple values:

ostringstream oss;
int id = 7;
string name = "Alice";
double score = 98.5;
oss << "Student #" << id << " (" << name << "): " << fixed << setprecision(1) << score;
cout << oss.str() << endl;
// Student #7 (Alice): 98.5

Using stringstream to parse strings

stringstream works in reverse too — you can extract values from a string:

#include <sstream>
using namespace std;

string data = "42 3.14 hello";
istringstream iss(data);

int n;
double d;
string word;

iss >> n >> d >> word;
// n = 42, d = 3.14, word = "hello"

This is handy for parsing structured text without reaching for a full parser.

Practical Examples

Building a filename with a number

#include <string>
using namespace std;

int frameNumber = 15;
string filename = "frame_" + to_string(frameNumber) + ".png";
// "frame_15.png"

Reading a number from user input

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

int main() {
    string input;
    cout << "Enter a number: ";
    cin >> input;

    try {
        int n = stoi(input);
        cout << "Double it: " << n * 2 << endl;
    } catch (...) {
        cout << "That's not a number." << endl;
    }
    return 0;
}

Splitting a comma-separated string of numbers

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

int main() {
    string csv = "10,20,30,40,50";
    vector<int> numbers;

    stringstream ss(csv);
    string token;

    while (getline(ss, token, ',')) {
        numbers.push_back(stoi(token));
    }

    for (int n : numbers) cout << n << " ";
    cout << endl;  // 10 20 30 40 50
    return 0;
}

Common Mistakes

Using + to concatenate int and string directly

int n = 5;
// string s = "Number: " + n;  // WRONG — undefined behaviour
string s = "Number: " + to_string(n);  // Correct

Trusting atoi’s return value
atoi("hello") returns 0. So does atoi("0"). You can’t tell them apart. Use stoi() with exception handling.

Forgetting that stoi stops at the first non-digit
stoi("12.5") returns 12 (it stops at the decimal point). Use stod() for floating-point strings.

Overflow with stoi for large numbers
stoi("99999999999") throws out_of_range because the number doesn’t fit in an int. Use stoll() for large values.

Quick Reference

Related Articles

• C++ Variables and Data Types — the types you’ll be converting between
• C++ String Handling — more string operations beyond conversion
• C++ User Input with cin — getting strings from the keyboard to convert
• C++ Exception Handling — properly handling stoi failures

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