ostringstream

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.

Exercise 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.

Show example solution

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

int main() {
    std::ostringstream oss;
    oss << "Result: " << 42;
    std::cout << oss.str();
    return 0;
}

Test yourself

Match each piece to its role

ostringstream

Writing to a string
<<

Data writing
.str()

Getting the string
oss << 42

Writing a number

Complete the str() call

std::string result = oss.____();

Show answer

std::string result = oss.str();

What does ostringstream oss; oss << 42; oss.str() return?
```
std::ostringstream oss;
oss << 42;
oss.str();
```
- "oss"
- 42
- '4'
- "42"
Show answer

"42" — str() returns the number as a string: "42".

Next lesson: stringstream

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