重載運算符以允許qsort在C++中工作

Question

我在C++中編寫了一段代碼，然後決定將其更改爲面向對象和除我的qsort之外的所有函數。我已經擡頭看到它是重載操作員，但我無法弄清楚如何做到這一點。任何人都可以請幫忙？

#include <iostream> 
#include <fstream> 
#include <string.h> 
using namespace std; 

class OpAmps { 
private: 
    char Name[20]; 
    unsigned int PinCount; 
    double SlewRate; 
public: 
    void Enter(OpAmps&, unsigned long&); 
    void Save(const OpAmps*, unsigned long); 
    void Load(OpAmps*, unsigned long&); 
    void Sort(OpAmps*, unsigned long); 
    int SortName(const void*, const void*); 
    int SortSlewRate(const void*, const void*); 
    void Display(const OpAmps*, unsigned long); 
}; 

#define DATABASE_MAX 10 
#define DATABASE_FILENAME "database.txt" 

int main() 
{ 
    OpAmps OpAmp[DATABASE_MAX]; 
    OpAmps Menu; 
    unsigned long database_length = 0; 
    char UserInput; 
    while (1) { 
     cout << endl; 
     cout << "Op-amp database menu" << endl; 
     cout << "--------------------" << endl; 
     cout << "1. Enter a new op-amp into the database" << endl; 
     cout << "2. Save the database to disk" << endl; 
     cout << "3. Load the database from disk" << endl; 
     cout << "4. Sort the database" << endl; 
     cout << "5. Display the database" << endl; 
     cout << "6. Exit from the program" << endl << endl; 
     cout << "Enter your option: "; 
     cin >> UserInput; 
     cout << endl; 
     switch(UserInput) { 
      case '1': 
      Menu.Enter(OpAmp[database_length], database_length); 
      break; 
      case '2': 
      Menu.Save(OpAmp, database_length); 
      break; 
      case '3': 
      Menu.Load(OpAmp, database_length); 
      break; 
      case '4': 
      Menu.Sort(OpAmp, database_length); 
      break; 
      case '5': 
      Menu.Display(OpAmp, database_length); 
      break; 
      case '6': 
      return 0; 
      default: 
      cout << "Invalid entry" << endl << endl; 
      break; 
     } 
    } 
} 

void OpAmps::Enter(OpAmps& Op, unsigned long& length) 
{ 
    if (length == DATABASE_MAX) { 
     cout << "The database is full" << endl; 
    } 
    else { 
     cout << "Add new data" << endl; 
     cout << "------------" << endl; 
     cout << "Enter op-amp name: "; 
     cin >> Op.Name; 
     cout << "Enter number of pins: "; 
     cin >> Op.PinCount; 
     cout << "Enter slew rate: "; 
     cin >> Op.SlewRate; 
     cout << endl; 
     length++; 
    } 
} 

void OpAmps::Save(const OpAmps* Op, unsigned long length) 
{ 
    fstream output_file; 
    output_file.open(DATABASE_FILENAME, ios::out); 
    if(output_file.good()) { 
     output_file << length << endl << endl; 
     for (unsigned long i=0;i<length;i++) { 
      output_file << Op[i].Name << endl; 
      output_file << Op[i].PinCount << endl; 
      output_file << Op[i].SlewRate << endl << endl; 
     } 
    } 
    output_file.close(); 
} 

void OpAmps::Load(OpAmps* Op, unsigned long& length) 
{ 
    fstream input_file; 
    input_file.open(DATABASE_FILENAME, ios::in); 
    if(input_file.good()) { 
     input_file >> length; 
     for (unsigned long i=0;i<length;i++) { 
      input_file >> Op[i].Name; 
      input_file >> Op[i].PinCount; 
      input_file >> Op[i].SlewRate; 
     } 
    } 
    input_file.close(); 
} 

void OpAmps::Sort(OpAmps* Op, unsigned long length) 
{ 
    char UserInput; 
    cout << endl; 
    cout << "Sorting options" << endl; 
    cout << "---------------" << endl; 
    cout << "1. To sort by name" << endl; 
    cout << "2. To sort by slew rate" << endl; 
    cout << "3. No sorting" << endl << endl; 
    cout << "Enter your option: "; 
    cin >> UserInput; 
    cout << endl; 
    switch(UserInput) { 
     case '1': 
     cout<<"sortName"<<endl; 
     qsort(Op,length,sizeof(OpAmps),SortName); 
     break; 
     case '2': 
     cout<<"sortslew"<<endl; 
     qsort(Op,length,sizeof(OpAmps),SortSlewRate); 
     break; 
     case '3': 
     return; 
     default: 
     cout << "Invalid entry" << endl << endl; 
     break; 
    } 
} 

int SortName(const void *First, const void* Second) 
{ 
    return strcmp(((OpAmps *) First)->Name, ((OpAmps *) Second)->Name); 
} 

int SortSlewRate (const void *First, const void* Second) 
{ 
    return (int) ((((OpAmps *) First)->SlewRate > ((OpAmps *) Second)->SlewRate)? 1 : -1); 
} 

void OpAmps::Display(const OpAmps* Op, unsigned long length) 
{ 
    if (length == 0) { 
     cout << "No elements in the database" << endl; 
    } 
    else { 
     cout << endl; 
     for (unsigned long i=0;i<length;i++) { 
      cout << "Name: " << Op[i].Name <<endl; 
      cout << "Number of Pins: " << Op[i].PinCount << endl; 
      cout << "Slew Rate: " << Op[i].SlewRate << endl; 
      cout << endl; 
     } 
    } 
} 

乾杯傢伙：d XX

Answer 1

我認爲在這種情況下，你的問題是，你都用相同的名字定義不同的功能。你已經完成了SortName和SortSlewRate。

一個定義表示「SortName」和「SortSlewRate」是OpAmps類的成員函數，但是在您的代碼中，SortName和SortSlewRate是全局函數。

qsort是一個C函數，它需要函數指針而不是成員函數指針。

既然你的函數是全局函數，但是訪問OpAmps類的私有成員，你應該把'friend'關鍵字放在它們前面。

試着改變你的聲明，這個...

class OpAmps { 
private: 
    char Name[20]; 
    unsigned int PinCount; 
    double SlewRate; 
public: 
    void Enter(OpAmps&, unsigned long&); 
    void Save(const OpAmps*, unsigned long); 
    void Load(OpAmps*, unsigned long&); 
    void Sort(OpAmps*, unsigned long); 
    void Display(const OpAmps*, unsigned long); 

    friend int SortName(const void*, const void*); 
    friend int SortSlewRate(const void*, const void*); 
}; 

所有這一切說，你會更好使用std ::排序，而不是快速排序，你會得到類型安全的優勢，在某些情況下，由於編譯器可以爲您提供優化，所以性能更高。

你的定義是這樣的

class OpAmps { 
private: 
    char Name[20]; 
    unsigned int PinCount; 
    double SlewRate; 
public: 
    void Enter(OpAmps&, unsigned long&); 
    void Save(const OpAmps*, unsigned long); 
    void Load(OpAmps*, unsigned long&); 
    void Sort(OpAmps*, unsigned long); 
    void Display(const OpAmps*, unsigned long); 

    friend bool SortName(const OpAmps &, const OpAmps &); 
    friend bool SortSlewRate(const OpAmps &, const OpAmps &); 
}; 

你可以使用 「排序」 像這樣...

sort(Op, Op + length,SortName); 

和

sort(Op,Op + length,SortSlewRate); 

注意，排序是一點點不同的，函數返回true（小於）或false（等於或大於）而不是-1（小於），0（等於），-1（大） r比），並且它們不傳遞指針，它們被傳遞給引用。

你會像這樣定義它們。

bool SortName(const OpAmps &First, const OpAmps &Second) 
{ 
    return strcmp(First.Name, Second.Name) < 0; 
} 

bool SortSlewRate (const OpAmps &First, const OpAmps &Second) 
{ 
    return First.SlewRate < Second.SlewRate; 
} 

最後，去更類似於C++，您可以將您的

char Name[20]; 

到

string Name; 

這將簡化您的SortName功能和反對骯髒的緩衝區溢出漏洞提供的安全性。

bool SortName(const OpAmps &First, const OpAmps &Second) 
{ 
    return First.Name < Second.Name; 
}