Multi-File Programs

Learn to organize code across multiple source files.

Prerequisites

Forward Declarations Not completed

Why use multiple files?

As programs grow larger, putting everything in one file becomes problematic:

Hard to navigate: Thousands of lines in one file are difficult to manage
Team collaboration: Multiple people can't easily work on the same file
Code reuse: Functions in one program can't be easily used in another
Compilation time: Changing one line requires recompiling the entire file
Organization: Related functions should be grouped together

Basic multi-file program structure

Let's start with a simple example. Instead of one large file, we'll split our program into multiple files:

main.cpp (main program file)

#include <iostream>

// Function declarations (forward declarations)
double add(double a, double b);
double subtract(double a, double b);
void displayAdditionResult(double result);
void displaySubtractionResult(double result);

int main()
{
    double x = 10.0;
    double y = 3.0;

    double sum = add(x, y);
    double difference = subtract(x, y);

    displayAdditionResult(sum);
    displaySubtractionResult(difference);

    return 0;
}

math_operations.cpp (mathematical functions)

#include <iostream>

// Function definitions
double add(double a, double b)
{
    return a + b;
}

double subtract(double a, double b)
{
    return a - b;
}

void displayAdditionResult(double result)
{
    std::cout << "Addition result: " << result << std::endl;
}

void displaySubtractionResult(double result)
{
    std::cout << "Subtraction result: " << result << std::endl;
}

How to compile this program

Our exercises handle this so you don't have to! When you are more advanced, using command line tools should be one of the skills you acquire.

To compile a multi-file program, you compile all source files together:

g++ main.cpp math_operations.cpp -o my_program

Or separately and then link:

g++ -c main.cpp              # Creates main.o
g++ -c math_operations.cpp   # Creates math_operations.o
g++ main.o math_operations.o -o my_program  # Links object files

Output:

Addition result: 13
Subtraction result: 7

The compilation and linking process

Here are the steps of how this program was compiled:

Preprocessing: Each .cpp file is preprocessed (includes processed, macros expanded)
Compilation: Each preprocessed file is compiled to object code (.o files)
Linking: Object files are combined into a single executable

main.cpp ──► preprocessing ──► compilation ──► main.o ──┐
                                                        ├──► linking ──► executable
math.cpp ──► preprocessing ──► compilation ──► math.o ──┘

Function declarations across files

When using functions from other files, you need forward declarations:

calculator.cpp

#include <iostream>

// Declarations for functions defined in other files
double calculateCircleArea(double radius);
double calculateCircleCircumference(double radius);

int main()
{
    // Use functions defined in geometry.cpp
    double radius = 5.0;
    std::cout << "Circle (r=" << radius << "):" << std::endl;
    std::cout << "  Area: " << calculateCircleArea(radius) << std::endl;
    std::cout << "  Circumference: " << calculateCircleCircumference(radius) << std::endl;
    
    return 0;
}

geometry.cpp

// Function definitions for geometric calculations

double calculateCircleArea(double radius)
{
     double PI = 3.14159;
    return PI * radius * radius;
}

double calculateCircleCircumference(double radius)
{
     double PI = 3.14159;
    return 2 * PI * radius;
}

Output:

Circle (r=5):
  Area: 78.5398
  Circumference: 31.4159

Real world example: Library management system

Here's how you might organize a library system:

library_system/
├── main.cpp              # Main program
├── book_operations.cpp   # Add, remove, search books
├── patron_operations.cpp # Patron management
├── checkout_system.cpp   # Check out/in books
├── report_generation.cpp # Generate various reports
└── data_validation.cpp   # Input validation functions

Each file would contain related functions, making the system easy to navigate and maintain.

Benefits of multiple files

1. Organization and maintainability

Consider the following program - a calculator that takes user input, validates it, performs mathematical operations, and displays results:

// Clear file structure:
// main.cpp        - program entry point and main logic
// calculations.cpp - mathematical functions
// input_output.cpp - user interface functions
// validation.cpp   - input validation functions

2. Selective recompilation

When you change one file, only that file needs to be recompiled:

## If only calculations.cpp changed:
g++ -c calculations.cpp      # Only recompile this file
g++ main.o calculations.o input_output.o validation.o -o program  # Re-link

3. Code reuse

Functions in separate files can be reused in multiple programs:

// math_utils.cpp can be used in multiple programs
// calculator.cpp, scientific_calculator.cpp, graphing_app.cpp

4. Team development

Different team members can work on different files simultaneously.

Best practices for multi-file programs

// 1. Organize by functionality
// math.cpp      - mathematical operations
// input.cpp     - user input functions
// display.cpp   - output formatting functions
// main.cpp      - main program logic

// 2. Use descriptive file names
student_records.cpp  // Clear purpose
validation.cpp       // Clear purpose
// vs
utils.cpp           // Too vague
stuff.cpp           // Meaningless

// 3. Keep main.cpp focused on program flow
int main()
{
    initializeProgram();
    processUserInput();
    displayResults();
    cleanup();
    return 0;
}

// 4. Group related functions in same file
// In geometry.cpp:
double calculateCircleArea(double radius);
double calculateCircleCircumference(double radius);
double calculateSphereVolume(double radius);