Multi-file Code

Multi-file Code

As coding projects increase in size, it helps to organize logic, functions, and classes into separate files.

File Contents: multiple .cpp

The example below shows how organize code across different files in a project that would be composed of the following files:

• main.cpp is the file that contains the int main() {} function.
• factorial.cpp contains a supporting function factorial() called by main.cpp.
• print.cpp also contains a supporting function print() called by main.cpp.

main.cpp

// #include <bits/stdc++.h> 
#include <iostream>
#include<string> 

// forward declaration
int factorial(int); 
void print(std::string);

using namespace std;
int main() { 

  int num = 7;
  int numFactorial = factorial(num);

  // print result
  string stringToPrint = "Factorial of [" + to_string(num) + "] is: " + to_string(numFactorial);
  print(stringToPrint);

} 

The file above calculates the factorial and constructs a string to be passed for printing. The file contains a forward declaration of the functions factorial() and print(), however these functions are in seperate files factorial.cpp and print.cpp.

factorial.cpp

#include <iostream>
using namespace std; 
  
// Recursive factorial program 
int factorial(int n) { 
  if (n == 1) { 
    return 1;
  }
  return n * factorial(n - 1); 
} 

The function above calculates the factorial of a number recursively. The factorial of a number is computed as n! = 1 * 2 * 3 * ... * n.

print.cpp

#include <iostream>
#include<string> 
using namespace std; 
  
void print(string str) { 
    cout << str << "\n"; 
} 

The function above prints a string to the standard output.

If you click Run, main.cpp should successfully execute with the following output.

// Factorial of [7] is: 5040

Header File: .h

We've previously used functions such as cin, cout, ifstream, ofstream, .lenght() that have no forward declaration, but are contained in header files such as <iostream>, <fstream>, and <string>. We can also create our own header file, which we demonstrate in the example below.

• function.h is a header file that declares the supporting functions factorial() and print().

function.h

#ifndef FUNCTION_H 
#define FUNCTION_H 

int factorial(int); 
void print(std::string);
  
#endif 

The file above is a header file that contains the following elements:

• #ifndef ... #endif wrapper that executes what is inclosed if the header function hasn't already been defined. This is to avoid compiling the contents of the file more than once.
• #define FUNCTION_H is a macro that declares the contents of the global variable FUNCTION_H. During compilation, all declarations of #include "function.h" will be replaced with the function declarations contained within.
• int factorial(int); void print(std::string); are forward declarations of the functions in factorial.cpp and print.cpp.

We will also need to make a slight update to main.cpp by inserting #include "function.h" (and remove the forward declarations of functions).

// main.cpp 
#include <iostream>
#include<string> 

// custom header file
#include "function.h" 

using namespace std;
int main() { 

  int num = 7;
  int numFactorial = factorial(num);

  // print result
  string stringToPrint = "Factorial of [" + to_string(num) + "] is: " + to_string(numFactorial);
  print(stringToPrint);

} 

If you click Run, main.cpp should successfully execute with the following output.

// Factorial of [7] is: 5040

An advantage of the header file approach, is that we can add forward declarations to the header file (i.e. in a centralized location) rather than update all files to include the forward declarations (of which could be many lines long).

References:

• C Header Files - tutorialspoint.com