Type conversion
- C++ course
- Intermediate
- Lesson 97/116
- 5 min
Learn type conversion with stringstream.
Converting a string to a number
The safe way to convert a string to a number is to use istringstream. The >> operator converts the number in the string to the desired type.
Example: std::istringstream iss("42"); int n; iss >> n;. Here the string "42" becomes an int and is stored in n. If the string has non-numeric characters >> fails; so checking the stream state after conversion is a good habit. This method is an alternative to functions like std::stoi and offers more control.
Converting a number to a string
To convert a number to a string you use ostringstream. Write the number to the stream with << and get the string with .str().
Example: std::ostringstream oss; oss << 42; std::string s = oss.str();. Here the number 42 becomes the string "42". This is the safe way to embed a number in a string. The std::to_string(42) function does the same but ostringstream offers formatting options. If you want to combine two numbers in one string you can write oss << a << " " << b; in a single stream.
Practice Convert string to number, add 8, write back
The starting code has a text string containing "42". Convert this string to an int with std::istringstream. Add 8. The result will be 50. Convert the number back to a string with std::ostringstream and print it with std::cout.
Starter code
#include <iostream>
#include <sstream>
#include <string>
int main() {
std::string text = "42";
// Convert to number, add 8, print the result
return 0;
}Sample solution
#include <iostream>
#include <sstream>
#include <string>
int main() {
std::string text = "42";
std::istringstream iss(text);
int num;
iss >> num;
num += 8;
std::ostringstream oss;
oss << num;
std::cout << oss.str();
return 0;
}Expected output
50
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Convert a string to a number
std::istringstream iss("____"); int n; iss >> n;std::istringstream iss("42"); int n; iss >> n;Explanation: The string "42" becomes a number.
2 Match each conversion direction
| Term | Meaning |
|---|---|
istringstream >> n |
String to number |
ostringstream << n |
Number to string |
.str() |
Getting the string |
>> |
Extracting from stream |
3 Which is used to convert an int to a string?
int n = 42;- std::ostringstream Correct answer
- std::ifstream
- std::vector
- std::cout
Explanation: ostringstream converts a number to a string.