How C++ Programs Work

A C++ source file is text that follows the language grammar. The compiler parses that text, checks types and syntax, and produces object code. A linker then connects object files and required library code into an executable. This two-stage model explains why a missing semicolon is a compile error while a declared-but-undefined function often becomes a linker error.

Execution is another stage. The operating system loads the executable and begins at main. A program can therefore compile correctly and still fail while running, or run to completion and calculate the wrong answer. Naming the stage of failure is the first debugging skill.

Every hosted C++ program has a main function. Braces contain its body, statements normally end with semicolons, and return 0 reports successful completion. The std::cout object writes characters to standard output. The insertion operator sends each value on its right into that stream.

#include <iostream>

int main()
{
  std::cout << "First line\n";
  std::cout << "Answer: " << 6 * 7 << '\n';
  return 0;
}

First line
Answer: 42

The #include directive makes declarations from a header available before compilation. The iostream header declares standard stream facilities. std:: qualifies a name from the standard-library namespace and prevents collisions with names from your program or another library.

Avoid a global using namespace std; in course examples. It saves a few characters but imports a large set of names into the current scope. Writing std::cout, std::string, and std::vector keeps ownership visible and scales better as a program gains dependencies.

Run the original program once, make one small change, predict its effect, and run again. This edit-build-observe loop connects source to behavior. If several changes are made together, a surprising result has several possible causes.

Diagnostics are evidence, not a score. Begin with the earliest useful error because later messages may be consequences. Copy the exact message, inspect the named location and nearby punctuation, then test one correction.