ostringstream
- C++ course
- Intermediate
- Lesson 95/116
- 5 min
Learn to write to strings with ostringstream.
Building a string with ostringstream
std::ostringstream builds a string by combining pieces. You write strings, numbers and variables in sequence with the << operator.
Example: std::ostringstream oss; oss << "Age: " << 25;. Here the oss stream receives the string "Age: " then the number 25. You get the resulting string with oss.str(): std::string result = oss.str(); gives "Age: 25". This is the clean way to gather multiple pieces into one string; it is more flexible than concatenating manually with +.
Getting the string with str()
The .str() method returns all data written to an ostringstream as a single string. You can use this string anywhere.
Example: oss << "Name: " << name << " Age: " << age; std::cout << oss.str();. This combines four pieces and prints them. The .str() call does not change the stream content; you can call it as many times as you want. To clear the stream you write oss.str("");. This flexibility makes ostringstream a powerful tool for everyday output formatting.
Practice Build a string and print it
Create a std::ostringstream. Write the string "Result: " and the number 42 with <<. Get the resulting string with .str() and print it with std::cout.
Starter code
#include <iostream>
#include <sstream>
#include <string>
int main() {
// Build the string and print it
return 0;
}Sample solution
#include <iostream>
#include <sstream>
#include <string>
int main() {
std::ostringstream oss;
oss << "Result: " << 42;
std::cout << oss.str();
return 0;
}Expected output
Result: 42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each piece to its role
| Term | Meaning |
|---|---|
ostringstream |
Writing to a string |
<< |
Data writing |
.str() |
Getting the string |
oss << 42 |
Writing a number |
2 Complete the str() call
std::string result = oss.____();std::string result = oss.str();Explanation: .str() returns the stream's string.
3 What does ostringstream oss; oss << 42; oss.str() return?
std::ostringstream oss;
oss << 42;
oss.str();- "oss"
- 42
- '4'
- "42" Correct answer
Explanation: str() returns the number as a string: "42".