By Anubhav Agarwal 
In C++, you convert a string to an int with the std::stoi() function from the <string> header: stoi("42") returns the int 42. It handles leading whitespace, a sign, and stops at the first non-digit character.
C++ also converts strings to numbers with stringstream, atoi() for C-style strings, strtol(), and a manual digit loop. This lesson shows all five ways, how each one handles invalid input, and the same conversion in plain C.
How Do You Convert a String to an Int in C++?
Call std::stoi() with the string, and it returns the parsed int.
#include <iostream>
#include <string>
using namespace std;
int main() {
string age_input = "27";
int age = stoi(age_input);
cout << age + 1; // Output: 28
return 0;
}
The arithmetic works because age is a real int after the call, not text.
5 Ways to Convert a String to an Int
1) The stoi() Function
stoi() parses an optional sign and digits, ignores leading whitespace, and stops at the first character that is not part of a number. An optional second argument reports where parsing stopped, and a third sets the base.
#include <iostream>
#include <string>
using namespace std;
int main() {
cout << stoi(" -350") << "\n"; // Output: -350
cout << stoi("42km") << "\n"; // Output: 42
cout << stoi("ff", nullptr, 16); // Output: 255
return 0;
}
2) A stringstream
The stringstream class from <sstream> reads formatted values out of a string the same way cin reads input. It is useful when a string holds several numbers.
#include <iostream>
#include <sstream>
#include <string>
using namespace std;
int main() {
stringstream reader("2026 7 21");
int year, month, day;
reader >> year >> month >> day;
cout << year + month + day; // Output: 2054
return 0;
}
3) The atoi() Function
atoi() from <cstdlib> converts a C-style string (const char*). On a std::string, call c_str() first. It returns 0 for invalid input, with no way to tell that apart from a real "0".
#include <iostream>
#include <cstdlib>
#include <string>
using namespace std;
int main() {
string code = "8080";
cout << atoi(code.c_str()) << "\n"; // Output: 8080
cout << atoi("oops"); // Output: 0
return 0;
}
4) The strtol() Function
strtol() from <cstdlib> converts to a long and sets an end pointer to the first unparsed character, so you can check how much of the string was a valid number without exceptions.
#include <iostream>
#include <cstdlib>
using namespace std;
int main() {
const char* text = "1500ms";
char* end;
long value = strtol(text, &end, 10);
cout << value << "\n"; // Output: 1500
cout << end; // Output: ms
return 0;
}
5) A Manual Digit Loop (Without stoi)
Converting without stoi() is a common exercise: multiply the result by 10 and add each digit's value, using the fact that '7' - '0' equals 7.
#include <iostream>
#include <string>
using namespace std;
int main() {
string digits = "934";
int number = 0;
for (char c : digits) {
number = number * 10 + (c - '0');
}
cout << number + 1; // Output: 935
return 0;
}
How stoi() Handles Invalid Input
stoi() throws an std::invalid_argument exception when the string starts with no digits, and std::out_of_range when the number does not fit in an int. Wrap the call in a try/catch to handle user input safely.
#include <iostream>
#include <string>
#include <stdexcept>
using namespace std;
int main() {
string user_input = "abc";
try {
int value = stoi(user_input);
cout << value;
} catch (const invalid_argument& e) {
cout << "not a number"; // Output: not a number
}
return 0;
}
When to Use Each Method
| Method | Use it when |
|---|---|
stoi() | A std::string holds one number; you want exceptions on bad input |
stringstream | The string holds several values to read in sequence |
atoi() | Legacy C-style strings where 0-on-failure is acceptable |
strtol() | You need error checking without exceptions, or the leftover text |
| Manual loop | Interview practice, or custom parsing rules |
Examples of Converting a String to an Int
1) Summing Numbers From Text Input
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main() {
vector<string> prices = {"250", "499", "120"};
int total = 0;
for (const string& p : prices) total += stoi(p);
cout << total; // Output: 869
return 0;
}
2) Parsing a Version Number
#include <iostream>
#include <string>
using namespace std;
int main() {
string version = "14.2";
size_t dot = version.find('.');
int major = stoi(version.substr(0, dot));
int minor = stoi(version.substr(dot + 1));
cout << major << " " << minor; // Output: 14 2
return 0;
}
3) Reading a Number and Its Unit
#include <iostream>
#include <string>
using namespace std;
int main() {
string distance = "42km";
size_t parsed;
int value = stoi(distance, &parsed);
cout << value << " " << distance.substr(parsed); // Output: 42 km
return 0;
}
Learn More About String to Int Conversion
String to Int in C
C has no stoi(). Use atoi() for quick conversions or strtol() when you need error detection, both from <stdlib.h>. The manual digit loop also works unchanged in C.
String to Long, Float, and Double
The same family covers other types: stol() and stoll() for long and long long, stof() and stod() for float and double. stod("3.14") returns the double 3.14.
Can You Cast a String to an Int?
No. (int)text and static_cast<int>(text) do not compile, because a string is an object, not a number in disguise. Conversion always goes through a parsing function.
Converting an Int Back to a String
The reverse direction uses std::to_string(): to_string(42) returns the string "42".
Key Takeaways for String to Int in C++
- stoi() - the standard way; handles whitespace, signs, other bases, and stops at the first non-digit.
- Errors -
stoi()throwsinvalid_argumentorout_of_range;atoi()silently returns 0. - strtol() - exception-free error checking through its end pointer.
- stringstream - reads several numbers out of one string in sequence.
- No casting - a cast from string to int does not compile; use a parsing function.
Parsed numbers usually end up in a container next. Read our lesson on initializing a vector in C++ for the ways to set one up.