从内置类型到自定义类的转换 [英] Conversion from built in types to Custom Classes

问题描述

我有一个自定义类，它充当一个称为Integer的int，我想告诉编译器如何自动将某些类型转换为Integer，这样我就可以避免一遍又一遍地键入相同的内容，

I have a custom class that acts as an int called Integer, I would like to tell compiler how to convert certain types to Integer automatically so that I can avoid typing same thing over and over again,

someCall(Integer(1), Integer(2));

将成为

someCall(1,2);

我已经在google上搜索过，但我能找到的只是做相反的事情，将Integer转换为int

I've googled but all I could find is to do the oposite, casting Integer to int I would like to accomplish the opposite.

推荐答案

写一个采用 int 的构造函数，例如：

Write a constructor that takes int, as:

class Integer
{
   public:
       Integer(int);
};

如果类 Integer 具有此构造函数，那么您可以执行以下操作：

If the class Integer has this constructor, then you can do this:

void f(Integer);

f(Integer(1)); //okay 
f(1);          //this is also okay!

原因是当您写 f（1），然后会自动调用 Integer 的构造函数，该构造函数接受单个类型为 int 的参数。临时，然后将该临时传递给函数！

The explanation is that when you write f(1), then the constructor of Integer that takes a single argument of type int, is automatically called and creates a temporary on the fly and and then that temporary gets passed to the function!

现在假设您想做相反的事情，即将 Integer 类型的对象传递给函数需要 int ：

Now suppose you want to do exactly the opposite, that is, passing an object of type Integer to a function takes int:

 void g(int); //NOTE: this takes int!

 Integer intObj(1);
 g(intObj); //passing an object of type Integer?

要使上述代码正常工作，您需要做的是，在类为：

To make the above code work, all you need is, define a user-defined conversion function in the class as:

class Integer
{
   int value;
   public:
       Integer(int);
       operator int() { return value; } //conversion function!
};

因此，当您传递 Integer 转换为使用 int 的函数，然后转换函数被调用，对象隐式转换为 int 然后传递该函数作为参数。您也可以这样做：

So when you pass an object of type Integer to a function which takes int, then the conversion function gets invoked and the object implicity converts to int which then passes to the function as argument. You can also do this:

int i = intObj; //implicitly converts into int
                //thanks to the conversion function!   

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