Function return value from form unit and non-form unit

问题

Previously I am able to get the result, but took it further to broaden my understanding its workings.

Unit1.h

#ifndef Unit1H
#define Unit1H
//---------------------------------------------------------------------------
#include <System.Classes.hpp>
#include <Vcl.Controls.hpp>
#include <Vcl.StdCtrls.hpp>
#include <Vcl.Forms.hpp>
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published:    // IDE-managed Components
TButton *Button1;
TButton *Button2;
TLabel *Label1;
TLabel *Label2;
void __fastcall Button1Click(TObject *Sender);
void __fastcall Button2Click(TObject *Sender);
private:    // User declarations
public:     // User declarations
    __fastcall TForm1(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
#endif

Unit1.cpp

//---------------------------------------------------------------------------

#include <vcl.h>
#pragma hdrstop

#include "Unit1.h"
#include "unit2.h"
#include "Unit3.h"

//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------

int lengthOfYard;
int widthOfYard;
int areaOfYard;

__fastcall TForm1::TForm1(TComponent* Owner)
: TForm(Owner)
{
}

//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Click(TObject *Sender)
{
Form2 = new TForm2(this);
widthOfYard = getWidth();
lengthOfYard = getLength();
//widthOfYard = 15;
//lengthOfYard = 17;
Form2->ShowModal();
}

//---------------------------------------------------------------------------
void __fastcall TForm1::Button2Click(TObject *Sender)
{
areaOfYard= FindArea(lengthOfYard,widthOfYard);
Label2->Caption = "\nYour yard is "+ String(areaOfYard) +" square feet\n\n";
}

//---------------------------------------------------------------------------

unit2.h

//---------------------------------------------------------------------------

#ifndef Unit2H
#define Unit2H
//---------------------------------------------------------------------------
#include <System.Classes.hpp>
#include <Vcl.Controls.hpp>
#include <Vcl.StdCtrls.hpp>
#include <Vcl.Forms.hpp>
//---------------------------------------------------------------------------
class TForm2 : public TForm
{
__published:    // IDE-managed Components
private:    // User declarations
public:     // User declarations
    __fastcall TForm2(TComponent* Owner);
    int getLength();
    int getWidth();
};
//---------------------------------------------------------------------------
extern PACKAGE TForm2 *Form2;
//---------------------------------------------------------------------------
#endif

unit2.cpp

//---------------------------------------------------------------------------

#include <vcl.h>
#pragma hdrstop

#include "Unit2.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm2 *Form2;

//---------------------------------------------------------------------------
__fastcall TForm2::TForm2(TComponent* Owner)
: TForm(Owner)
{
}
//---------------------------------------------------------------------------

int getLength()
{
    return 15;
}

//---------------------------------------------------------------------------

int getWidth()
{
    return 17;
}

//---------------------------------------------------------------------------

Unit3.h

//---------------------------------------------------------------------------
#ifndef Unit3H
#define Unit3H
//---------------------------------------------------------------------------

// declare the function here in the header file.

int FindArea(int length, int width); //function prototype

#endif//--------------------------------------------------------------------    -------

unit3.cpp

//---------------------------------------------------------------------------

#pragma hdrstop

#include "Unit3.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)

int FindArea(int l, int w)
{
    return l * w;
}

I got the error like

[bcc32 Error] Unit1.cpp(31): E2268 Call to undefined function 'getLength'

and

[bcc32 Error] Unit1.cpp(32): E2268 Call to undefined function 'getWidth'

I cannot figure it as I compared these new functions' declarations and definitions with "FindArea" function.

回答1:

You're calling getLength() and getWidth() from within Unit1.cpp.

In that translation unit, these two functions aren't visible ("they're in" Unit2.cpp. Take a look at What is external linkage and internal linkage?).

You have two options:

add
```
extern int getLength();
extern int getWidth();
```
at the beginning of Unit1.cpp file. The extern keyword indicates external linkage (i.e. the definition of the function may be expected to come from some other C++ file). The compiler will trust that you have implemented the function somewhere. If the linker cannot find it, you will get a linker error.
declare the two functions in Unit2.h and include this header file in Unit1.cpp (for further details: C++ extern keyword on functions. Why no just include the header file?).

BUT

you should note that your getLength() function isn't the implementation of the TForm2::getLength() member function.

To define TForm2::getLength() you have to write:

int TForm2::getLength()
{
  return 15;
}

(you should also include Unit2.h inside Unit1.cpp)

The syntax to call TForm2::getLength() is:

Form2->getLength()

Usually a member function that doesn't access private/protected data members doesn't make much sense (you should prefer a free function).

来源：https://stackoverflow.com/questions/31067798/function-return-value-from-form-unit-and-non-form-unit

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