Input & Output Streams

Streams are C++'s unified model for I/O. Once you learn cin/cout, the same syntax reads files, writes files, builds strings, and even talks over sockets in libraries.

The Stream Family

They all share the same << / >> interface. Master one, master all.

Output: cout << ...

int x = 42;
double pi = 3.14;
string name = "Asha";
cout << "x=" << x << ", pi=" << pi << ", name=" << name << "\n";

Each << returns the stream, which is why you can chain.

Newlines: "\n" vs endl

cout << "fast\n";       // newline only — fast
cout << "slow" << endl; // newline + FLUSH (forces write to OS)

Inside loops or when you print millions of lines (competitive programming!), use "\n". Use endl only when you actually need a flush.

Input: cin >> ...

int a, b;
cin >> a >> b;          // reads two whitespace-separated numbers

>> skips leading whitespace, then reads tokens. It stops at the first whitespace (space, tab, newline).

Reading Strings — Two Modes

string word;
cin >> word;            // reads one word — stops at space
string line;
getline(cin, line);     // reads to end of line

The Classic Bug: >> Then getline

int n;
string line;
cin >> n;               // leaves '\n' in the buffer
getline(cin, line);     // ❌ reads empty string!
// FIX:
cin >> n;
cin.ignore();           // discard the leftover newline
getline(cin, line);     // ✅

cin.ignore(numeric_limits::max(), '\n') is the safer, full version.

Detecting End of Input

cin is "truthy" while it can still read.

int x;
while (cin >> x) {       // loops until EOF or read fails
    cout << x * 2 << "\n";
}

This pattern handles input streams of unknown length — perfect for files piped on stdin.

Formatted Output —

#include <iomanip>
cout << fixed << setprecision(3) << 1.0/3;   // 0.333
cout << setw(10) << "hi";                    // pads to width 10 (right-aligned)
cout << left << setw(10) << "hi" << "
";     // left-aligned: "hi
"
cout << setfill('0') << setw(4) << 42;       // "0042"
cout << hex << 255 << "\n";                  // "ff"
cout << boolalpha << true;                    // "true"

setw resets after one output; fixed, hex, etc. stay set until changed.

Files: ofstream & ifstream

#include <fstream>
#include <string>
using namespace std;
// Write
ofstream out("data.txt");
out << "Hello\n" << 42 << "\n";
out.close();             // optional — destructor closes too
// Read
ifstream in("data.txt");
string line;
while (getline(in, line)) {
    cout << line << "\n";
}

Always check the file opened:

ifstream in("missing.txt");
if (!in) {
    cerr << "Could not open file\n";
    return 1;
}

Stringstream: Parse a String Like Input

#include <sstream>
string data = "42 3.14 hello";
stringstream ss(data);
int    n;
double d;
string s;
ss >> n >> d >> s;       // n=42, d=3.14, s="hello"

Indispensable when you need to split a line into tokens.

Speed Tips (Competitive Programming)

By default, cin/cout synchronise with C's stdio — slower. To go fast:

ios_base::sync_with_stdio(false);
cin.tie(nullptr);

These two lines at the top of main can speed I/O up to 10x. After this, don't mix cin/cout with scanf/printf in the same program.

Cheat-Sheet

I/O looks small, but it's where most beginner bugs hide. Master getline + cin.ignore() + while(cin >> x) and you've handled 90% of input formats you'll meet.