重載運算符以允許qsort在C ++中工作

Question

我用C ++編寫了一段代碼，然后決定將其更改為面向對象的代碼，以及除qsort以外的所有函數。 我已經看過它與操作符重載有關，但是我不知道該怎么做。 誰能幫忙嗎？

#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;
        }
    }
}

干杯的家伙：D xx

Answer 1

我認為在這種情況下，您的問題是您使用相同的名稱定義了不同種類的函數。 您已經對SortName和SortSlewRate完成了此操作。

一個定義說“ 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C函數，需要函數指針而不是成員函數指針。

由於您的函數旨在用作全局函數，但可以訪問OpAmps類的私有成員，因此應在它們前面放置“ friend”關鍵字。

嘗試將聲明更改為此...

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*);
};

綜上所述，使用std :: sort而不是qsort會更好，您將獲得類型安全的優勢，並且在某些情況下，由於編譯器可以為您進行優化，因此性能更高。

您的定義如下所示

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 &);
};

你會這樣使用“排序” ...

    sort(Op, Op + length,SortName);

和

    sort(Op,Op + length,SortSlewRate);

請注意，排序略有不同，函數返回true（小於）或false（等於或大於）而不是-1（小於），0（等於），-1（大於），它們不是t傳遞了指針，它們被傳遞了引用。

您可以這樣定義它們。

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;
}

最后，要使用更多的C ++，您可以切換

char Name[20];

至

string Name;

這將簡化您的SortName函數並提供安全性，以防止討厭的緩沖區溢出錯誤。

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