c++ - 表单单元和非表单单元的函数返回值

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。

单元1.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";
}

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

单元2.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

单元2.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;
}

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

单元3.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;
}

我得到了这样的错误

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

和

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

当我将这些新函数的声明和定义与“FindArea”函数进行比较时，我无法理解。

Answer 1

您正在从 Unit1.cpp 中调用 getLength() 和 getWidth()。

在那translation unit ，这两个函数是不可见的(“它们在”Unit2.cpp。看看 What is external linkage and internal linkage?)。

你有两个选择:

• 添加

  extern int getLength();
  extern int getWidth();
  

  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 中声明这两个函数，并将此头文件包含在 Unit1.cpp 中(更多详细信息: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()，您必须这样写:

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

(您还应该在 Unit1.cpp 中包含 Unit2.h)

调用 TForm2::getLength() 的语法是:

Form2->getLength()

通常不访问私有(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 )。