Running First Program

The canonical “hello world” — an int main() that prints a line — exercises the full edit → compile → link → run loop and confirms the toolchain works end to end.

Why it matters

This is the smoke test for your whole setup (setting-up-the-environment): if it compiles and runs, your compiler, standard library, linker, and PATH are all wired correctly. It also introduces three things you’ll use forever — #include, the special main function, and stream output via input-output-iostream.

How it works

main is the program’s entry point; the OS calls it, and its return value is the process exit code (0 = success, nonzero = failure — visible to shells and CI).

• #include <iostream> pulls in declarations for std::cout and std::endl.
• std::cout << x chains because operator<< returns the stream by reference.
• return 0; is optional in main only — falling off the end implicitly returns 0; every other non-void function must return.

Phase	Command (GCC)	Produces
Compile + link	g++ -std=c++20 hello.cpp -o hello	executable hello
Run	./hello	stdout text
Check exit code	echo $?	0

Example

#include <iostream>
 
int main() {
    std::cout << "Hello, world!" << '\n';
    return 0;   // exit code 0 = success
}

$ g++ -std=c++20 -Wall hello.cpp -o hello
$ ./hello
Hello, world!
$ echo $?          # the exit code main returned
0

Prefer '\n' over std::endl here: std::endl also flushes the buffer, which is needless overhead per line in tight loops.

Pitfalls

• void main() is non-standard — the signature is int main() or int main(int argc, char** argv). Some compilers tolerate void; don’t rely on it.
• Forgetting std:: — cout without using or qualification won’t compile; avoid using namespace std; in headers (it leaks names).
• #include <iostream.h> — the .h form is a pre-standard header that no longer exists; use <iostream>.
• Running before compiling succeeds — a missing linker step (only -c) yields an .o, not a runnable binary.

See also

• how-c-works-compilation-model
• input-output-iostream