What is the typical top-to-bottom structure of a C++ source file?

Includes → namespace declaration → preprocessor #defines → global variables → function definitions → main().

A C++ source file is read top-to-bottom by the compiler, and names must be declared before they are used. That dictates a conventional layout:

#include <iostream>      // 1. include files
using namespace std;     // 2. namespace declaration

#define LIMIT 50         // 3. preprocessor defines

int A = 0;               // 4. global variables

int fib(int n) {         // 5. function definitions
    if (n > 1) return fib(n-1) + fib(n-2);
    else return 1;
}

int main(void) {         // 6. main function definition
    int n;               //    (local variables live inside it)
    cout << "Enter n: ";
    cin >> n;
    cout << "Fib(" << n << ") = " << fib(n) << endl;
    return 0;
}

Why this order matters:

• The preprocessor (#include, #define) runs first, pasting in headers and substituting macros before compilation.
• Helper functions like fib() appear above main() so they're already declared when main() calls them (otherwise you'd need a forward declaration/prototype).
• main() is the program's entry point and conventionally comes last.

Tip: This layout is essentially identical to C — C++ inherits the same single-pass, declare-before-use model.