函数 return 来自表单单元和非表单单元的值

Question

以前我能够得到结果，但进一步扩展了我对它的工作原理的理解。

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;
}

我收到类似

的错误

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

和

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

我无法理解，因为我将这些新函数的声明和定义与 "FindArea" 函数进行了比较。

Answer 1

您正在从 Unit1.cpp.

中调用 getLength() 和 getWidth()

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

你有两个选择：

添加
```
extern int getLength();
extern int getWidth();
```
在 Unit1.cpp 文件的开头。 extern 关键字表示外部链接（即函数的定义可能来自其他一些 C++ 文件）。编译器会相信你已经在某个地方实现了这个函数。如果链接器找不到它，您将收到链接器错误。
在Unit2.h中声明两个函数，并在Unit1.cpp中包含这个头文件（更多细节：C++ extern keyword on functions. Why no just include the header file?）。

但是

您应该注意，您的 getLength() 函数不是 TForm2::getLength() 成员函数的实现。

要定义 TForm2::getLength() 你必须写：

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

（你还应该在 Unit1.cpp 中包含 Unit2.h）

调用TForm2::getLength()的语法是：

Form2->getLength()

通常不访问 private/protected 数据成员的成员函数没有多大意义（您应该更喜欢 free function）。

函数 return 来自表单单元和非表单单元的值

Function return value from form unit and non-form unit

c++

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