函数从窗体单位和非窗体单位返回值 [英] Function return value from form unit and non-form unit
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问题描述
以前，我能够得到结果，但进一步扩大了我的理解其工作。
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管理的组件
TButton * Button1;
TButton * Button2;
TLabel * Label1;
TLabel * Label2;
void __fastcall Button1Click（TObject * Sender）;
void __fastcall Button2Click（TObject * Sender）;
private：//用户声明
public：//用户声明
__fastcall TForm1（TComponent * Owner）;
};
// -------------------------------------------- -------------------------------
extern PACKAGE TForm1 * Form1;
// -------------------------------------------- -------------------------------
#endif
Unit1.cpp
-------------------------------------------------- --------------------
#include< vcl.h>
#pragma hdrstop
#includeUnit1.h
#includeunit2.h
#includeUnit3.h
// ---------------------------------------------- -----------------------------
#pragma package（smart_init）
#pragma resource* .dfm
TForm1 * Form1;
// -------------------------------------------- -------------------------------
int lengthOfYard;
int widthOfYard;
int regionOfYard;
__fastcall TForm1 :: TForm1（TComponent * Owner）
：TForm（Owner）
{
}
// --- -------------------------------------------------- ----------------------
void __fastcall TForm1 :: Button1Click（TObject * Sender）
{
Form2 = new TForm2 （这个）;
widthOfYard = getWidth（）;
lengthOfYard = getLength（）;
// widthOfYard = 15;
// lengthOfYard = 17;
Form2-> ShowModal（）;
}
// ----------------------------------- ----------------------------------------
void __fastcall TForm1 :: Button2Click （TObject * Sender）
{
areaOfYard = FindArea（lengthOfYard，widthOfYard）;
Label2-> Caption =\\\
Your yard is+ String（areaOfYard）+square feet\\\
\\\
;
}
// ----------------------------------- ----------------------------------------
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：//用户声明
public：//用户声明
__fastcall TForm2（TComponent * Owner）;
int getLength（）;
int getWidth（）;
};
// -------------------------------------------- -------------------------------
extern PACKAGE TForm2 * Form2;
// -------------------------------------------- -------------------------------
#endif
unit2.cpp
-------------------------------------------------- --------------------
#include< vcl.h>
#pragma hdrstop
#includeUnit2.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
// ------- -------------------------------------------------- ------------------
//在头文件中声明这里的函数。
int FindArea（int length，int width）; // function prototype
#endif // ---------------------------------- ---------------------------------- -------
unit3.cpp
-------------------------------------------------- ------------------------
#pragma hdrstop
#includeUnit3.h
//------------------------------------------- -------------------------------
#pragma package（smart_init）
int FindArea（int l，int w）
{
return l * w;
}
我得到的错误是
[bcc32错误] Unit1.cpp（31）：E2268调用未定义函数'getLength'
和
[bcc32错误] Unit1.cpp（32）：E2268调用未定义的函数'getWidth'
我不能理解为我将这些新函数的声明和定义与 c> getLength（）
$
getWidth（）在翻译单元中，这两个函数不可见（他们是in Unit2.cpp 。查看什么是外部链接和内部链接
-
add
extern int getLength（）;
extern int getWidth（）; Unit1.cpp 文件开头的
。 extern 关键字表示外部链接（即，函数的定义可能预期来自某个其他C ++文件）。编译器将相信你已经在某处实现了函数。如果链接器找不到它，你将得到一个链接器错误。
-
BUT
您应该注意到您的 getLength（）函数不是 TForm2 :: getLength（）成员函数。
要定义 TForm2 :: getLength 您必须写：
int TForm2 :: getLength（）
{
return 15;
}
（您还应该包括 Unit2.h Unit1.cpp ）
调用 TForm2的语法： ：getLength（）是：
Form2-> getLength（）
通常，不访问私有/受保护的数据成员的成员函数没有什么意义（你应该更喜欢免费功能）。
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.
解决方案 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 in C++).
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).
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