C++中的数据转换

用户定义的用户定义数据类型可满足其需求，编译器不支持此类数据类型的自动类型转换，因此，如果需要，用户需要自行设计转换例程。

在不兼容的数据类型之间进行数据转换时，可能会出现3种情况：

• 从原始数据类型到用户定义类型的转换：要执行此转换，其想法是在对象创建期间使用构造函数执行类型转换。下面是将int转换为用户定义的数据类型的示例：

例子：

C++

// C++ program to illustrate the
// type-conversion
#include 
using namespace std;
 
// Time Class
class Time {
    int hour;
    int mins;
 
public:
    // Default Constructor
    Time()
    {
        hour = 0;
        mins = 0;
    }
 
    // Parameterized Constructor
    Time(int t)
    {
        hour = t / 60;
        mins = t % 60;
    }
 
    // Function to print the value
    // of class variables
    void Display()
    {
        cout << "Time = " << hour
             << " hrs and "
             << mins << " mins\n";
    }
};
 
// Driver Code
int main()
{
    // Object of Time class
    Time T1;
    int dur = 95;
 
    // Conversion of int type to
    // class type
    T1 = dur;
    T1.Display();
 
    return 0;
}

---

输出

Time = 1 hrs and 35 mins

• 类对象到原始数据类型的转换：在这种转换中， from类型是一个类对象，而to类型是原始数据类型。一个重载铸造运算符函数的正常形式，也称为转换函数。以下是相同的语法：

句法：

operator typename()
{
   // Code
}

• 现在，此函数将用户定义的数据类型转换为原始数据类型。例如，运算符double()将类对象转换为double类型，运算符int()将类类型对象转换为int类型，依此类推。下面是说明相同内容的程序：

例子：

C++

// C++ program to illustrate the
// above conversion
#include 
using namespace std;
 
// Tie Class
class Time {
    int hrs, mins;
 
public:
    // Constructor
    Time(int, int);
 
    // Casting operator
    operator int();
 
    // Destructor
    ~Time()
    {
        cout << "Destructor is called."
             << endl;
    }
};
 
// Function that assigns value to the
// member variable of the class
Time::Time(int a, int b)
{
    hrs = a;
    mins = b;
}
 
// int() operator is used for Data
// conversion of class to primitive
Time::operator int()
{
    cout << "Conversion of Class"
         << " Type to Primitive Type"
         << endl;
 
    return (hrs * 60 + mins);
}
 
// Function perfomrs type conversion
// from the Time class type object
// to int data type
void TypeConversion(int hour, int mins)
{
    int duration;
 
    // Create Time Class  object
    Time t(hour, mins);
 
    // Conversion OR duration = (int)t
    duration = t;
    cout << "Total Minutes are "
         << duration << endl;
 
    // Conversion from Class type to
    // Primitive type
    cout << "2nd method operator"
         << " overloading " << endl;
 
    duration = t.operator int();
 
    cout << "Total Minutes are "
         << duration << endl;
 
    return;
}
 
// Driver Code
int main()
{
    // Input value
    int hour, mins;
    hour = 2;
    mins = 20;
 
    // Function call to illustrate
    // type conversion
    TypeConversion(hour, mins);
 
    return 0;
}

输出

Conversion of Class Type to Primitive Type
Total Minutes are 140
2nd method operator overloading 
Conversion of Class Type to Primitive Type
Total Minutes are 140
Destructor is called.

输出：

Conversion of Class Type to Primitive Type
Total Minutes are 140
2nd method operator overloading 
Conversion of Class Type to Primitive Type
Total Minutes are 140
Destructor is called.

现在，该函数会将矢量转换为标量大小。运算符double()可以用作：

double len = double(S1);
            Or,
double len = S1;
where S1 is an object of type vector.

一种类类型到另一种类类型的转换：在这种类型中，一种类类型被转换成另一种类类型。可以通过2种方式完成：

1，使用构造函数

2.使用重载铸造运算符

1，使用构造函数

在Destination类中，我们使用构造方法

//Objects of different types 
ObjectX=ObjectY;
Here ObjectX is Destination object and ObjectY is source object

例子：

C++

#include
using namespace std;
//cgs system
class CGS
 {
     int mts; //meters
     int cms; //centimeters
     public:
       void showdata()
   {
       cout<<"Meters and centimeters in CGS system:";
       std::cout << mts<<" meters "<

输出

Meters and centimeters in CGS system:9 meters 10 centimeters
feet and inches in FPS system:30 feet 4 inches

2.使用重载铸造运算符

// Objects of different types
objectX = objectY;

• 这里我们在源类中使用重载转换运算符，即在源类中重载目标类

请参见下面的示例，在该示例中，我们分别有两个类Time和Minute，并将一个类Time转换为另一个Minute类。

在下面的示例中，分钟类是目标类，时间类是源类

所以我们需要在源类中重载目标类

这里我们不应该告诉返回类型，但是我们返回重载的类对象

即在不指定返回类型的情况下返回值

C++

// C++ program to illustrate the
// above conversion
#include 
using namespace std;
//minutes class
class Minute {
     
 
public:
   int mins;
    // Constructors
    Minute()
    {
        mins = 0;
    }
 
    // Function to print the value of
    // hours and minutes
    void show()
    {
        cout << "\nTotal Minute : " << mins << endl;
    }
};
 
// Time Class
class Time {
    int hr, mins;
 
public:
    // Constructors
    Time(int h, int m)
    {
        hr = h;
        mins = m;
    }
    Time()
    {
        cout << "\nTime's Object Created";
    }
    operator Minute  () //overloading minute class
    {
        Minute m;
        m.mins = (hr * 60) + mins;
        return m;
    } //driver code
 
    // Function to print the value of
    // hours and minutes
    void show()
    {
        cout << "Hour: " << hr << endl;
        cout << "Minute : " << mins << endl;
    }
};
 
// Minutes Class
int main()
{
    Time T1(3,40);
    Minute m;
    m=T1; //mintue class is destination and Time class is source class
    T1.show();
    m.show();
    return 0;
}

输出

Hour: 3
Minute : 40

Total Minute : 220

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