C++ Builder Tutorials

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
See also:

  Functions in C++