[C++] How to Use String Streams (stringstream) | Reading and Writing Strings in Memory

2025年12月31日

Introduction

In C++, there are frequent situations where you want to convert numbers to strings, convert strings to numbers, or split a space-separated string into individual words. By using “String Streams” provided by the standard library <sstream> header, you can achieve these tasks using the exact same intuitive syntax as cout and cin with the << / >> operators.

In this article, I will explain the usage of the two main string stream classes:

std::ostringstream: Used for writing (formatting) various data into a string.
std::istringstream: Used for reading (parsing) a string to decompose it into various data types.

1. std::ostringstream: Constructing (Writing) Strings

ostringstream (output string stream) appends various data (numbers, strings, etc.) passed via the << operator into an internal string buffer. Finally, calling the .str() member function allows you to retrieve the constructed std::string.

Sample Code

#include <iostream>
#include <sstream> // String streams
#include <string>

using namespace std;

int main() {
    ostringstream oss;
    
    string item = "Apple";
    int price = 120;
    int count = 3;
    
    // 1. Write data of various types to the stream using the << operator
    oss << "Item: " << item
        << ", Price: " << price << " yen"
        << ", Qty: " << count
        << ", Total: " << price * count << " yen";
        
    // 2. Retrieve the constructed string with .str()
    string result = oss.str();
    
    cout << result << endl;
    
    return 0;
}

Execution Result

Item: Apple, Price: 120 yen, Qty: 3, Total: 360 yen

2. std::istringstream: Decomposing (Reading) Strings

istringstream (input string stream) is created based on an existing string. Using the >> operator, you can extract data separated by spaces or newlines into variables one by one, just as if you were reading from cin.

Sample Code

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

using namespace std;

int main() {
    string source_data = "Sato 35 Development";
    
    // 1. Initialize istringstream with the string you want to read
    istringstream iss(source_data);
    
    string name, department;
    int age;
    
    // 2. Read space-separated data into variables sequentially using the >> operator
    iss >> name >> age >> department;
    
    // 3. Verify the read results
    cout << "Name: " << name << endl;
    cout << "Age: " << age << " years old" << endl;
    cout << "Dept: " << department << endl;

    return 0;
}

Execution Result

Name: Sato
Age: 35 years old
Dept: Development

Summary

In this article, I explained the basic usage of string streams in C++.

ostringstream: Used to construct a single formatted string by combining data of various types.
istringstream: Used to decompose a string separated by spaces or other delimiters into individual data pieces.
stringstream: There is also a class that can handle both reading and writing with a single object.

By mastering string streams, you can write complex string formatting and parsing processes safely and concisely using the intuitive interface common to cout and cin.

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