Your first program: main, cout and namespaces

The smallest useful program

#include <iostream>

int main() {
    std::cout << "Namaste, C++!\n";
    return 0;
}

Let us read it line by line.

• #include <iostream> brings in the input/output part of the standard library. The standard library is the large set of ready-made code that comes with every C++ compiler.
• int main() starts the main function. A function is a named block of code. The operating system calls main when the program starts. int means main gives back a whole number.
• The curly braces { } mark the body of main: the statements that run, top to bottom.
• std::cout is the "character output" stream: text sent to it appears in the terminal. << means "send this into the stream".
• "\n" is the newline character: it moves the cursor to the next line.
• return 0; ends main and tells the operating system the program finished without error. Any other number usually means an error. (In main only, you may leave this line out; the compiler then returns 0 for you.)
• Every statement ends with a semicolon ;. Forgetting it is the most common beginner error.

Chaining output

#include <iostream>

int main() {
    std::cout << "Train: " << 12951 << '\n';
    std::cout << "Fare: Rs " << 1450.50 << '\n';
    std::cout << 7 + 5 << '\n';
}

You can chain many << in one statement. Text in double quotes is a string; numbers are printed as numbers, and expressions like 7 + 5 are calculated first. A single character uses single quotes, like '\n'.

std::endl versus '\n'

std::endl also ends the line, but it additionally flushes the stream: it forces the text out to the screen immediately. Flushing is slow when you print many lines, so prefer '\n' and use std::endl only when you really need the text shown at once (for example just before a long calculation).

Namespaces and std::

A namespace is a named group of names, so two libraries can both have something called "count" without a clash. Everything in the standard library lives in the namespace std. std::cout means "the cout that belongs to std". :: is called the scope resolution operator.

#include <iostream>
using namespace std;   // fine in tiny practice files, avoid in real projects

int main() {
    cout << "No std:: needed here\n";
}

Comments

// A comment until the end of the line
/* A comment that can
   span many lines */
int main() { }

Reading compiler errors

Compiler messages look scary but always give a file name and line number, like hello.cpp:4:40: error: expected ';' before '}' token. Fix the FIRST error first; later errors are often caused by the first one. A missing semicolon is often reported on the next line.