C++ How to Convert String to Int Easily

In C++, you can convert a string to an int using the stoi function like this: int num = std::stoi(str); where str is your string.

Examples

Input"123"

Output123

Input"0"

Output0

Input"-456"

Output-456

How to Think About It

To convert a string to an int, think of reading the number characters one by one and turning them into a real number. The stoi function does this by checking the string and extracting the integer value it represents.

Algorithm

1

Take the input string that contains digits.

2

Use a built-in function to parse the string and convert it to an integer.

3

Store or return the integer value.

Code

cpp

#include <iostream>
#include <string>

int main() {
    std::string str = "123";
    int num = std::stoi(str);
    std::cout << num << std::endl;
    return 0;
}

Output

123

Dry Run

Let's trace converting the string "123" to an int using stoi.

1

Input string

str = "123"

2

Call stoi

num = std::stoi("123")

3

Result

num = 123 (integer)

Step	String	Integer Value
1	"123"	N/A
2	"123"	123
3	N/A	123

Why This Works

Step 1: Using std::stoi

The stoi function reads the string and converts the numeric characters into an integer value.

Step 2: Handling signs

If the string starts with a minus sign, stoi correctly converts it to a negative integer.

Step 3: Error handling

stoi throws an exception if the string does not represent a valid integer.

Alternative Approaches

Using stringstream

cpp

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

int main() {
    std::string str = "123";
    std::stringstream ss(str);
    int num;
    ss >> num;
    std::cout << num << std::endl;
    return 0;
}

This method is more flexible but slightly slower than stoi.

Using atoi (legacy)

cpp

#include <iostream>
#include <cstdlib>

int main() {
    const char* cstr = "123";
    int num = std::atoi(cstr);
    std::cout << num << std::endl;
    return 0;
}

This is an older C-style method that does not throw exceptions but requires a C-string.

Complexity: O(n) time, O(1) space

Time Complexity

The conversion scans each character once, so it takes linear time relative to the string length.

Space Complexity

No extra space is needed besides a few variables, so it uses constant space.

Which Approach is Fastest?

stoi is generally faster and safer than stringstream or atoi because it is optimized and throws exceptions on errors.

Approach	Time	Space	Best For
std::stoi	O(n)	O(1)	Simple, safe, modern C++
stringstream	O(n)	O(1)	Flexible parsing, multiple types
atoi	O(n)	O(1)	Legacy code, C-style strings