Streams

Streams

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Prerequisites

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1. What is a Stream?

A stream in C++ is an abstraction for input/output operations.

[ Source ] → (Input Stream) → Program → (Output Stream) → [ Destination ]

            std::ios
               │
        ┌──────┴────────────┐
   std::istream         std::ostream
        │                   │
   std::ifstream        std::ofstream
   std::istringstream   std::ostringstream

All streams are built on top of std::ios

std::istream (Input Stream)

Used for input operations

Example

#include <iostream>

int x;
std::cin >> x;

std::cin is an instance of std::istream

std::cin >> x;        // extraction
std::cin.get();       // get character
std::cin.ignore();    // skip input
std::cin.peek();      // look ahead

std::ostream (Output Stream)

Used for output operations

Example

#include <iostream>

std::cout << "Hello";

std::cout is an instance of std::ostream

std::cout << x;       // insertion
std::cout.put('A');   // write character
std::cout.flush();    // force output

std::ifstream (File Input Stream)

Reads data from a file.

Example

#include <fstream>

std::ifstream file("data.txt");

int x;
file >> x;

if (!file)
{
    std::cout << "Failed to open file\n";
}

std::ofstream (File Output Stream)

#include <fstream>

std::ofstream file("output.txt");

file << "Hello file";

std::istringstream (String Input Stream)

Treats a string like an input stream.

Example

#include <sstream>

std::string data = "10 20 30";

std::istringstream iss(data);

int a, b, c;
iss >> a >> b >> c;

Useful for parsing strings

std::ostringstream (String Output Stream)

#include <sstream>

std::ostringstream oss;

oss << "Value: " << 10;

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

std::streambuf (Core of Streams)

The actual buffer layer behind all streams

[ Stream (iostream) ]
        ↓
[ streambuf (buffer) ]
        ↓
[ Device (file, console, string) ]

• controls actual I/O behavior
• handles buffering
• used for custom stream implementations

Example: Access buffer

std::streambuf* buf = std::cout.rdbuf();

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Redirect Output

#include <fstream>

std::ofstream file("log.txt");

std::streambuf* old = std::cout.rdbuf(file.rdbuf());
std::cout << "This goes to file\n";

std::cout.rdbuf(old);  // restore

2. Performance Considerations

Streams are slower than C-style I/O

std::cout << x;

vs

printf("%d", x);

• type safety
• formatting
• abstraction overhead

Disable sync with C I/O

std::ios::sync_with_stdio(false);
std::cin.tie(nullptr); // stop cout.flush

Huge speed boost for competitive programming

Type	Purpose
istream	input
ostream	output
ifstream	file input
ofstream	file output
istringstream	string input
ostringstream	string output
streambuf	buffer layer

3. Common Mistakes

❌ Mixing cin and getline

std::cin >> x;
std::getline(std::cin, str);  // ❌ leftover newline

✔️ Fix:

std::cin >> x;
std::cin.ignore();   // remove newline
std::getline(std::cin, str);

❌ Not flushing output when needed

std::cout << "Waiting...";

👉 May not appear immediately

✔️ Fix:

std::cout << "Waiting..." << std::flush;