c++ - 实现从一个类到另一个类的类型转换

Question

假设我有两个没有继承关系的类。例如:

class MyString
{
    private:
        std::string str;
};

class MyInt
{
    private:
        int num;
};

并且我希望能够使用常规转换将一个转换为另一个，例如 MyInt a = (MyInt)mystring(其中 mystring 属于 class MyString)。

如何完成这样的事情？

Answer 1

转换首先需要有意义。如果是这样，您可以实现自己的转换运算符，如下例所示:

#include <string>
#include <iostream>

class MyInt; // forward declaration

class MyString
{
    std::string str;
public:
    MyString(const std::string& s): str(s){}
    /*explicit*/ operator MyInt () const; // conversion operator
    friend std::ostream& operator<<(std::ostream& os, const MyString& rhs)
    {
        return os << rhs.str;
    }
};

class MyInt
{
    int num;
public:
    MyInt(int n): num(n){}
    /*explicit*/ operator MyString() const{return std::to_string(num);} // conversion operator
    friend std::ostream& operator<<(std::ostream& os, const MyInt& rhs)
    {
        return os << rhs.num;
    }
};

// need the definition after MyInt is a complete type
MyString::operator MyInt () const{return std::stoi(str);} // need C++11 for std::stoi

int main()
{
    MyString s{"123"};
    MyInt i{42};

    MyInt i1 = s; // conversion MyString->MyInt
    MyString s1 = i; // conversion MyInt->MyString

    std::cout << i1 << std::endl;
    std::cout << s1 << std::endl;
}

Live on Coliru

如果你将转换运算符标记为explicit，这是可取的(需要C++11或更高版本)，那么你需要显式转换，否则编译器会报错，比如

MyString s1 = static_cast<MyString>(i1); // explicit cast