C++ Builder: Function Library

If you have functions that may be used in several projects, the best place to put them would a formless unit, say a "library" unit called Funcs.

Short example

1. Start a new project, say FunctionTest, with a form named Form1.
2. In the menu, select File / New / Unit C++Builder
3. Save the unit as Funcs.

You declare the functions in Funcs.h (a "header" file) and put the implementation in Funcs.cpp. Proceed as follows:

1. In Funcs.h, add the following lines:

   int Summer(int a, int b);
   int Multiplier(int a, int b);

2. In Funcs.cpp, add:

   int Summer(int a, int b)
   { 
     return (a + b);
   }
   
   int Multiplier(int a, int b)
   {
     return (a * b);
   }

3. Before using the functions in your Unit1.cpp, you have to tell C++ Builder that Unit1.cpp "uses" Funcs.cpp:
   Open Unit1.cpp, in the menu select File / Use Unit... and select Funcs.cpp
   
   This adds the following line near the top of Unit1.cpp:
     #include "Funcs.h"
   
   From now on, you can use these functions in Unit1.cpp, for example:

   void __fastcall TForm1::Button1Click(TObject *Sender)
   {
     Label1->Caption = IntToStr(Summer(3, 5));
   }

 CBTFunc  : a library with useful functions

Start by creating a test project and next add a second unit without a form. But this time, save that unit as CBTFunc.cpp.

We want the following functions:

• ArrAverage() returns the average of an array of floating point numbers.
• Factorial() returns the factorial (n!) of an integer number.
  n! = 1 x 2 x 3...(n-2) x (n-1) x n
• ProgDir() returns the folder ("directory") where the application's exe-file is located.
• WeekDayStr() returns a string with the day of the week for a given date.
• WeekDayNr() returns the number of the day of the week for a given date.

Add the declarations and the implementations to CBTFunc:

1. Modify CBTFunc.h:

   #ifndef CBTFuncH
   #define CBTFuncH
   //------------------------------------------------------
   #endif
   #include <System.Classes.hpp>  // for the String class
   
   float ArrAverage(float arr[], int size);
   String ProgDir();
   String WeekDayStr(TDateTime D);
   int WeekDayNr(int y, int m, int d);

2. Modify CBTFunc.cpp:

   #pragma hdrstop
   
   #include "CBTFunc.h"
   #include <vcl.h>     // for the TApplication class
   //--------------------------------------------------------
   #pragma package(smart_init)
   
   float ArrAverage(float arr[], int size)
   {
     int i;
     float sum;
     sum = 0;
     for (i = 0; i < size; ++i) {
       sum += arr[i];
     }
     return (sum / size);
   }
   
   String ProgDir()
   {
     return ExtractFilePath(Application->ExeName);
   }
   
   String WeekDayStr(TDateTime D)
   {
     String Days[7] = {"Monday", "Tuesday", "Wednesday",
     "Thursday", "Friday", "Saturday", "Sunday"};
     int WeekDayNr;
     WeekDayNr = DayOfWeek(D); // returns integer 1 to 7
     return (Days[WeekDayNr - 1]); // Days[0] is "Monday"
   }

3. Don't forget to declare that Unit1.cpp uses CBTFunc.cpp:
   In the menu, select File / Use Unit... and select CBTFunc.cpp

After you have tested our examples, we advise you to back up the files to a safe location ;)
Next, feel free to add your own functions!

Use CBTFunc in any project

Copy-pasting the code is the worst form of code reuse. If you want to use your library with functions in another project:

1. Copy both files CBTFunc.h and CBTFunc.cpp to the folder of the other project.
2. Open your project in C++ Builder, in the menu select Project / Add to Project... and select CBTFunc.cpp.
3. Open the unit where you want to use the functions of CBTFunc, say in Unit1.cpp. In the menu, select File / Use Unit... and select CBTFunc.cpp

Example project

Here's a small project for testing. Start a new project, save the files, and next:

1. Copy both files CBTFunc.h and CBTFunc.cpp to the folder of your project.
2. Add a button and a TMemo component the form.
3. Create an event handler for the button:

   void __fastcall TForm1::Button1Click(TObject *Sender)
   {
     float nums[5] = {10, 20, 3.14, 6, 9};
     Memo1->Lines->Add("ProgDir: " + ProgDir());
     Memo1->Lines->Add("Average: " + FloatToStr(ArrAverage(nums, 5)));
     Memo1->Lines->Add("!5 = " + IntToStr(Factorial(5)));
     Memo1->Lines->Add("Week day: " + WeekDayStr(Date()));
   }

4. In the menu select Project / Add to Project... and select CBTFunc.cpp.
5. Open Unit1.cpp. In the menu, select File / Use Unit... and select CBTFunc.cpp