编译 C++Builder 6 项目时出现“未解决的外部”错误

Question

我在编译 C++Builder 6 线程项目时遇到了一些错误。我不知道如何解决它们。

Unit1.cpp：

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

#include <vcl.h>
#pragma hdrstop

#include "Unit1.h"

using namespace std;

int i1 = 0, i2 = 0;
T1 *Thread1, *Thread2;
bool Term1, Term2;
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
}

void __fastcall T1::Execute()
{
    do
        Synchronize (UpdateTimer);
    while (!Terminated);
}
//-----------------------------------------------
void __fastcall T1::UpdateTimer()
{
    Timer->Enabled = !Terminated;
}

//---------------------------------------------------------------------------
void __fastcall TForm1::FormCreate(TObject *Sender)
{
    ComboBox1->ItemIndex = 3;
    ComboBox2->ItemIndex = 3;
    Thread1 = new T1(false);
    Thread1->Timer = Timer1;
    Thread1->FreeOnTerminate = true;
    Term1 = false;
    Thread2 = new T1(true);
    Thread2->Timer = Timer2;
    Thread2->FreeOnTerminate = true;
    Term2 = false;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Timer1Timer(TObject *Sender)
{
    i1++;
    Label1->Caption = i1;
    Timer1->Enabled = false;

}
//---------------------------------------------------------------------------
void __fastcall TForm1::Timer2Timer(TObject *Sender)
{
    i2++;
    Label2->Caption = i2;
    Timer2->Enabled = false;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BRes1Click(TObject *Sender)
{
    if (!Term1)
        Thread1->Resume();
    else ShowMessage("Ïîòîê óæå çàâåðøåí");
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BRes2Click(TObject *Sender)
{
    if (!Term2)
        Thread2->Resume();
    else ShowMessage("Ïîòîê óæå çàâåðøåí");
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BTerm1Click(TObject *Sender)
{
    Thread1->Terminate();
    Term1 = true;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BTerm2Click(TObject *Sender)
{
    Thread2->Terminate();
    Term2 = true;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BSuspend1Click(TObject *Sender)
{
    if (!Term1)
        Thread1->Suspend();
    else ShowMessage("Ïîòîê óæå çàâåðøåí");
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BSuspend2Click(TObject *Sender)
{
    if (!Term2)
        Thread2->Suspend();
    else ShowMessage("Ïîòîê óæå çàâåðøåí");
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BSyncClick(TObject *Sender)
{
    if (Term1 || Term2)
        ShowMessage("Ïîòîê óæå çàâåðøåí");
    else
    {
        if (!Thread1->Suspended)
            Thread1->Suspend();
        if (!Thread2->Suspended)
            Thread2->Suspend();
        Timer1->Enabled = false;
        Timer2->Enabled = false;
        i1 = 0;
        i2 = 0;
        Thread1->Resume();
        Thread2->Resume();
    }

}
//---------------------------------------------------------------------------
void __fastcall TForm1::ComboBox1Change(TObject *Sender)
{
    if (Term1)
        ShowMessage("Ïîòîê óæå çàâåðøåí");
    else
        switch (ComboBox1->ItemIndex)
        {
            case 0: Thread1->Priority = tpIdle;          break;
            case 1: Thread1->Priority = tpLowest;        break;
            case 2: Thread1->Priority = tpLower;         break;
            case 3: Thread1->Priority = tpNormal;        break;
            case 4: Thread1->Priority = tpHigher;        break;
            case 5: Thread1->Priority = tpHighest;       break;
            case 6: Thread1->Priority = tpTimeCritical;  break;
        }

}
//---------------------------------------------------------------------------
void __fastcall TForm1::ComboBox2Change(TObject *Sender)
{
    if (Term2)
        ShowMessage("Ïîòîê óæå çàâåðøåí");
    else
        switch (ComboBox2->ItemIndex)
        {
            case 0: Thread2->Priority = tpIdle;          break;
            case 1: Thread2->Priority = tpLowest;        break;
            case 2: Thread2->Priority = tpLower;         break;
            case 3: Thread2->Priority = tpNormal;        break;
            case 4: Thread2->Priority = tpHigher;        break;
            case 5: Thread2->Priority = tpHighest;       break;
            case 6: Thread2->Priority = tpTimeCritical;  break;
        }
}
//---------------------------------------------------------------------------

Uni1.h

//---------------------------------------------------------------------------
#ifndef Unit1H
#define Unit1H
//---------------------------------------------------------------------------
#include <Classes.hpp>
#include <Controls.hpp>
#include <StdCtrls.hpp>
#include <Forms.hpp>
#include <ExtCtrls.hpp>
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published:    // IDE-managed Components
    TGroupBox *GroupBox1;
    TGroupBox *GroupBox2;
    TLabel *Label1;
    TButton *BRes1;
    TButton *BSuspend1;
    TButton *BTerm1;
    TLabel *Label2;
    TButton *BRes2;
    TButton *BSuspend2;
    TButton *BTerm2;
    TComboBox *ComboBox1;
    TComboBox *ComboBox2;
    TButton *BSync;
    TTimer *Timer1;
    TTimer *Timer2;
    void __fastcall FormCreate(TObject *Sender);
    void __fastcall Timer1Timer(TObject *Sender);
    void __fastcall Timer2Timer(TObject *Sender);
    void __fastcall BRes1Click(TObject *Sender);
    void __fastcall BRes2Click(TObject *Sender);
    void __fastcall BTerm1Click(TObject *Sender);
    void __fastcall BTerm2Click(TObject *Sender);
    void __fastcall BSuspend1Click(TObject *Sender);
    void __fastcall BSuspend2Click(TObject *Sender);
    void __fastcall BSyncClick(TObject *Sender);
    void __fastcall ComboBox1Change(TObject *Sender);
    void __fastcall ComboBox2Change(TObject *Sender);
private:    // User declarations
public:     // User declarations
    __fastcall TForm1(TComponent* Owner);
};

class T1 : public TThread
{
private:
    void _fastcall UpdateTimer();
protected:
    void _fastcall Execute();
public:
    __fastcall T1(bool CreateSuspended);
    TTimer *Timer;
};

//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
#endif

错误：

[Linker Error] Unresolved external 't1::' referenced from C:\USERS\ALEXEY\DESKTOP\CCC\UNIT1.OBJ
[Linker Error] Unresolved external '__fastcall T1::T1(bool)' referenced from C:\USERS\ALEXEY\DESKTOP\CCC\UNIT1.OBJ

Answer 1

您已经声明了一个类T1，其中声明了一个构造函数，但您实际上并没有实现该构造函数。这就是链接器所抱怨的。

您需要将该实现添加到您的.cpp 文件中，例如：

__fastcall T1::T1(bool CreateSuspended)
    : TThread(CreateSuspended)
{
    FreeOnTerminate = true; // <-- this should be moved in here
}

这将解决链接器错误。

---

话虽如此，您的代码中存在一些设计缺陷。

您正在使用CreateSuspended=false 创建Thread1 对象，它允许它在其构造函数完成后立即开始运行。但是在构造函数退出之前，您不会分配其 Timer 成员。因此线程可以在分配Timer 之前开始运行。在您的示例中此“有效”的唯一原因是因为您对 TThread::Synchronize() 的使用不会调用 T1::UpdateTimer()，直到 TForm1::FormCreate() ¹ 分配了 Timer 成员并退出

^{1：顺便说一句，您根本不应该在 C++ 中使用该事件，因为它可能导致未定义的行为。请改用 Form 的构造函数。}

但这不是一个值得依赖的好设计。由于您的线程类依赖于外部 TTimer 对象，您应该：

• 使用CreateSuspended=true 创建Thread1 对象，然后在完全初始化其成员后Resume() 该线程。这就是您对 Thread2 对象所做的事情。

• 将所需的TTimer 对象指针传递给每个线程的构造函数，例如：

class T1 : public TThread
{
private:
    TTimer *Timer;
    void _fastcall UpdateTimer();
protected:
    void _fastcall Execute();
public:
    __fastcall T1(bool CreateSuspended, TTimer *ATimer);
};

...

__fastcall T1::T1(bool CreateSuspended, TTimer *ATimer)
    : TThread(CreateSuspended), Timer(ATimer)
{
    FreeOnTerminate = true;
}

...

Thread1 = new T1(false, Timer1);
Thread2 = new T1(true, Timer2);

此外，您正在为两个线程设置FreeOnTerminate=true，但您没有为每个线程分配OnTerminate 事件处理程序，因此当您尝试访问时无法确定线程是否实际有效它们在您的按钮OnClick 事件处理程序中。如果线程因任何原因终止，包括意外崩溃，那么您将访问悬空指针。

此外，您不应该出于任何原因从该线程自己的Execute() 方法之外Suspend()'ing 一个正在运行的线程。您不知道暂停时线程的状态。它可能（在这种情况下很可能会）在Synchronize() 调用的中间，这可能导致等待暂停的Synchronize() 完成其工作的死锁。如果您想暂停工作线程，请改用单独的标志，甚至使用TEvent，线程可以在安全且合适的情况下定期检查。

---

最后，正如您所做的那样，在工作线程中的紧密循环中调用 TThread::Synchronize() 是对工作线程的完全浪费，因为它的 ALL工作仅在主 UI 线程中执行。你也可以完全摆脱你的TThread 类。使用TTimer 就足够了，或者使用PostMessage() 循环，或者TApplication::OnIdle 事件处理程序，或者TAction(Manager)::OnUpdate 事件处理程序等。有很多不同的方法来解决这个问题。